perf(db): add owner-prefixed hot-path indexes on JobApplications

Add composite indexes (OwnerUserId, IsDeleted) — for the tenant-scoped
list/board/stats/analytics queries that all filter !IsDeleted — and
(OwnerUserId, FollowUpAt) for the reminders surface. Every JobApplication
query is scoped by the OwnerUserId global filter first, so owner-prefixed
composites are the useful shape; the pre-existing single OwnerUserId index
is now a redundant prefix but kept to avoid churn.

Status is intentionally excluded: Pomelo maps the unbounded string column to
MariaDB longtext, which cannot be indexed without a prefix length.

Applied via the startup schema reconciler (StartupInitializationExtensions),
which is how this repo actually provisions schema/indexes on both providers
(SQLite: CREATE INDEX IF NOT EXISTS; MariaDB: MySqlIndexExists-guarded CREATE
INDEX) — NOT via EF migrations, whose committed ModelSnapshot is stale.
OnModelCreating also declares the indexes for model consistency.

Backend suite: 92/92 green.

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

Data/JobTrackerContext.cs

@@ -55,6 +55,20 @@ namespace JobTrackerApi.Data
         modelBuilder.Entity<JobApplication>()
             .HasIndex(j => j.OwnerUserId);
 
+        // Owner-prefixed composite indexes for the tenant-scoped hot paths. Every
+        // JobApplication query is scoped by the OwnerUserId global filter first, then
+        // filtered by IsDeleted (list/board/stats/analytics) or FollowUpAt (reminders).
+        // Status is intentionally excluded from the index because Pomelo maps the
+        // unbounded string column to longtext, which MariaDB cannot index without a
+        // prefix length. The actual index DDL is applied idempotently in
+        // StartupInitializationExtensions (this repo provisions schema via that
+        // reconciler, not via the EF ModelSnapshot, which is stale).
+        modelBuilder.Entity<JobApplication>()
+            .HasIndex(j => new { j.OwnerUserId, j.IsDeleted });
+
+        modelBuilder.Entity<JobApplication>()
+            .HasIndex(j => new { j.OwnerUserId, j.FollowUpAt });
+
         modelBuilder.Entity<Company>()
             .HasIndex(c => c.OwnerUserId);