make the repository layer engine-agnostic (#1127)

DatabaseContext handed every repository a raw better-sqlite3 handle alongside
drizzle, and three of them used it for retention queries built on datetime('now',
?) — a SQLite-only function. That handle is the one thing standing between the
repository layer and a second engine.

Drop it. The two time-based prunes compute their cutoff in JS against the
CURRENT_TIMESTAMP text format, which every engine writes the same way and which
compares correctly as a string; the health-history prune becomes a select of the
rows to keep followed by a NOT IN delete. All three turn async, so their two
callers await them.

Name the dialect rather than repeating a string literal, so adding an engine is
one edit instead of a search.

Tests built their schema through context.sqlite?.exec(). Optional chaining meant
removing the field type-checked cleanly and then silently created no tables, so
the fixture now owns exec() and a raw handle for direct assertions — schema setup
belongs to the test harness, not to the interface repositories consume.

No behaviour change, and no Postgres yet: this only removes the coupling that
would have to be undone first.

src/backend/tests/database/repositories/alert-repository.test.ts

@@ -17,7 +17,7 @@ describe("AlertRepository", () => {
   ): Promise<AlertRepository> {
     adapter = new TestSqliteDatabase();
     const context = await adapter.connect();
-    context.sqlite?.exec(`
+    adapter.exec(`
       CREATE TABLE users (
         id TEXT PRIMARY KEY,
         username TEXT NOT NULL,
@@ -229,7 +229,7 @@ describe("AlertRepository", () => {
     expect(unacknowledged.total).toBe(0);
 
     await repo.acknowledgeAllFirings("user-1");
-    repo.pruneFiringsOlderThan("user-1", 0);
+    await repo.pruneFiringsOlderThan("user-1", 0);
   });
 
   it("loads enabled rules and notification channels for the alert engine", async () => {