chore: document mariadb bootstrap and add deploy health checks

deploy/MARIADB.md

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+# MariaDB production bootstrap
+
+## What this app supports
+- SQLite for local/dev fallback
+- MariaDB/MySQL for production when configured with:
+  - `DATABASE_PROVIDER=mariadb`
+  - `JOBTRACKER_CONNECTION_STRING=...`
+
+## 1. Create database and user in MariaDB
+Run this against your MariaDB server/container:
+
+```sql
+CREATE DATABASE jobtracker CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+CREATE USER 'jobtracker'@'%' IDENTIFIED BY 'REPLACE_WITH_STRONG_PASSWORD';
+GRANT ALL PRIVILEGES ON jobtracker.* TO 'jobtracker'@'%';
+FLUSH PRIVILEGES;
+```
+
+## 2. Configure server `.env`
+Example:
+
+```env
+DATABASE_PROVIDER=mariadb
+JOBTRACKER_CONNECTION_STRING=server=mariadb;port=3306;database=jobtracker;user=jobtracker;password=REPLACE_WITH_STRONG_PASSWORD
+AUTH_JWT_KEY=replace_with_long_random_secret
+AUTH_ADMIN_EMAIL=you@example.com
+AUTH_ADMIN_PASSWORD=replace_with_strong_password
+APP_PUBLIC_BASE_URL=https://your-domain.example
+SUMMARIZER_BASE_URL=http://summarizer:8001
+```
+
+## 3. First startup
+On first deploy, the API will run EF migrations at startup.
+That means the MariaDB schema is created automatically as long as:
+- the DB exists
+- credentials are correct
+- the backend container can reach `mariadb`
+
+## 4. SQLite to MariaDB migration notes
+This repo does **not** yet include an automated data migration tool.
+If you already have real data in SQLite, recommended migration path is:
+
+1. Stop writes to the app
+2. Back up SQLite DB file
+3. Start MariaDB-backed environment on a staging copy
+4. Export/import the critical tables with a small migration script or one-time tool
+5. Validate users, companies, jobs, correspondence, attachments metadata
+6. Switch production `.env` to MariaDB
+
+## Tables you would likely want to migrate
+- `AspNetUsers`
+- `AspNetRoles`
+- `AspNetUserRoles`
+- `Companies`
+- `JobApplications`
+- `Correspondences`
+- `Attachments`
+- `JobEvents`
+- `GmailConnections`
+- `UserRuleSettings`
+
+## Important note on attachments
+Files are stored separately from DB rows.
+When moving environments, make sure you preserve:
+- `/data`
+- especially attachments and data-protection keys
+
+## Suggested validation checklist after switching
+- can log in with admin user
+- profile loads
+- jobs list loads
+- correspondence loads
+- attachments metadata still matches files on disk
+- summarizer works
+- reminders page works
+- admin/system page loads