chore(ops): add deployment backups restore docs and health checks

Closes the three operational blockers from the production readiness review.

deploy.sh now takes a database backup before it builds, stops or replaces
anything, and aborts the deploy if the backup fails — so no deploy proceeds
without a restore point. Dumps are gzipped and timestamped into
/opt/job-tracker/backups (override with BACKUP_DIR), so one deploy never
overwrites an earlier backup. Credentials come from the existing connection
string and travel via MYSQL_PWD, never on the command line, so they cannot reach
the process list or the deploy log. A dump that is empty or missing CREATE TABLE
is rejected, because a truncated file that looks like a restore point is worse
than none. SQLite deployments get their data volume tarred instead. Nothing is
ever deleted automatically; retention is documented as manual.

deploy/README.md documents backup creation, location, retention, database
restore, application rollback, and when to use which — restore and rollback kept
distinct, because a bad deploy usually needs only the rollback and restoring
would discard everything written since the dump.

Health checks now cover backend and frontend, which previously had none. GET
/health is anonymous, cheap, and deliberately does not touch the database: a
health check that queried MariaDB would restart a healthy backend whenever the
database blipped, and would hand out an unauthenticated way to probe database
availability. The backend image gains curl on the existing chromium apt layer,
since the aspnet runtime ships neither curl nor wget. frontend now waits for
backend to be healthy rather than merely started, because nginx proxies /api and
refuses to start when the upstream cannot be resolved.

Verified against real containers, no production data: backup from a seeded
MariaDB 11; restore into a clean MariaDB 11 with rows identical; bad credentials
and a missing connection string both abort non-zero and leave no partial file;
SQLite volume backup produces a readable archive; backend and frontend both
reach healthy; and a backend pointed at an unreachable database exits and is
reported unhealthy, so a broken deploy cannot present as a running stack.

Incidentally confirmed the earlier authorization work: with Auth:Require unset,
/health returns 200 while /api/jobapplications returns 401.

393 backend tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/README.md

@@ -100,3 +100,122 @@ If this app is going to be a real production service on Ubuntu:
 - confirm reminder and admin/system pages load
 - verify follow-up reminder emails are enabled only when intended and that links open the correct job/tab
 hat links open the correct job/tab
+
+---
+
+# Backups, restore and rollback
+
+**Database restore and application rollback are two different operations.** A bad deploy usually needs
+only the rollback. Restore the database only if the data itself is wrong or lost — it discards
+everything written since the dump.
+
+## Backup creation
+
+`deploy/deploy.sh` takes a backup **before** it builds, stops or replaces anything, and **aborts the
+deploy if the backup fails**. Nothing else in the deploy runs without a restore point.
+
+- **Location:** `/opt/job-tracker/backups` — override with `BACKUP_DIR`.
+- **Naming:** `jobtracker-<database>-<UTC timestamp>.sql.gz`, e.g.
+  `jobtracker-jobtracker-20260719T153759Z.sql.gz`. The timestamp makes every file unique, so a deploy
+  never overwrites an earlier backup.
+- **SQLite deployments** (`DATABASE_PROVIDER` unset or `sqlite`) get the data volume instead:
+  `jobtracker-sqlite-<UTC timestamp>.tar.gz`.
+- **Credentials** come from `JOBTRACKER_CONNECTION_STRING` and are passed via `MYSQL_PWD`, never on the
+  command line, so they cannot appear in the process list or the deploy log.
+- **Compression:** gzip. A small database compresses to a few KB.
+- **Verification:** the script rejects a dump that is empty or missing `CREATE TABLE`, because a
+  truncated file that *looks* like a restore point is worse than none.
+
+### Taking one by hand
+
+```bash
+BACKUP_DIR=/opt/job-tracker/backups
+mkdir -p "$BACKUP_DIR"
+MYSQL_PWD='<password>' mariadb-dump \
+  --host=127.0.0.1 --port=3306 --user=<user> \
+  --single-transaction --routines --events --quick \
+  jobtracker | gzip -c > "$BACKUP_DIR/jobtracker-manual-$(date -u +%Y%m%dT%H%M%SZ).sql.gz"
+```
+
+### Retention
+
+**Nothing is deleted automatically.** Backups accumulate in `BACKUP_DIR` until you remove them. Watch
+disk usage and prune deliberately — a suggested policy is to keep every backup for 30 days and one per
+month after that, but the script does not enforce it and will not delete your files.
+
+## Restoring the database
+
+Tested end-to-end against a clean MariaDB 11 container: dump taken from a seeded database, restored
+into an empty one, rows verified identical.
+
+```bash
+# 1. Stop the application so nothing writes during the restore.
+docker compose stop backend
+
+# 2. Restore. This REPLACES the current contents of the named database.
+gzip -dc /opt/job-tracker/backups/jobtracker-jobtracker-20260719T153759Z.sql.gz \
+  | MYSQL_PWD='<password>' mariadb --host=127.0.0.1 --port=3306 --user=<user> jobtracker
+
+# 3. Verify before starting anything.
+MYSQL_PWD='<password>' mariadb --host=127.0.0.1 --port=3306 --user=<user> jobtracker \
+  -e "SELECT COUNT(*) AS applications FROM JobApplications;"
+
+# 4. Start again.
+docker compose start backend
+```
+
+Restoring a **SQLite** deployment instead:
+
+```bash
+docker compose stop backend
+docker run --rm -v jobtracker_data:/data -v /opt/job-tracker/backups:/backup \
+  -e ARCHIVE_NAME=jobtracker-sqlite-20260719T153941Z.tar.gz \
+  alpine:3 sh -c 'rm -rf /data/* && tar xzf "/backup/$ARCHIVE_NAME" -C /data'
+docker compose start backend
+```
+
+## Restoring application containers (rollback)
+
+This is the usual fix for a bad deploy, and it **does not touch the database**.
+
+```bash
+cd /opt/job-tracker/app          # the deployment checkout
+git log --oneline -5             # find the last good commit
+git checkout <previous-commit>
+deploy/deploy.sh
+```
+
+`deploy.sh` takes a fresh backup first, so rolling back is itself protected.
+
+**Why a code rollback is safe here:** every Phase 4/5 migration is a no-op — the startup reconciler
+owns those tables — so reverting the code never leaves migration history ahead of the schema. The
+reconciler is additive and never drops a table holding rows, so the older code simply ignores the
+newer tables.
+
+**What a code rollback does not undo:** rows users created in the newer tables stay. That is usually
+what you want. If you additionally restore the database, those rows are lost — so restore only when
+the data is the problem.
+
+## Choosing between them
+
+| Symptom | Action |
+|---|---|
+| New version starts but behaves wrong | Rollback the code. Leave the database. |
+| Backend will not start; schema looks wrong | Rollback the code, then restore only if it still fails. |
+| Data is missing or corrupted | Restore the database from the most recent good dump. |
+| Deploy aborted before starting | Nothing to undo — the backup ran before any change. |
+
+## Health checks
+
+`backend` and `frontend` both report container health, so `docker compose ps` shows real state rather
+than merely "running".
+
+- **Backend:** `curl -fsS http://127.0.0.1:8080/health`. Anonymous, and deliberately **does not touch
+  the database** — a health check that queried MariaDB would restart a healthy backend whenever the
+  database blipped. `start_period` is 90s to cover first-boot schema reconciliation.
+- **Frontend:** `wget` against nginx on port 80.
+- `frontend` waits for `backend` to be *healthy*, not merely started, because nginx proxies `/api` to
+  it and refuses to start if the upstream cannot be resolved.
+
+A backend that cannot reach its database exits and is reported `unhealthy`, so a broken deploy does not
+present as a running stack.