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/test-support.ts

@@ -15,12 +15,31 @@ export class TestSqliteDatabase {
     this.context = {
       dialect: "sqlite",
       drizzle: drizzle(this.sqlite, { schema }),
-      sqlite: this.sqlite,
     };
 
     return this.context;
   }
 
+  /**
+   * Schema setup for tests. Lives on the fixture rather than on
+   * DatabaseContext, which is drizzle-only so that no repository can reach for
+   * engine-specific SQL.
+   */
+  /** Raw handle for assertions that read the database directly. Tests only. */
+  get raw(): Database.Database {
+    if (!this.sqlite) {
+      throw new Error("connect() must be called before raw access");
+    }
+    return this.sqlite;
+  }
+
+  exec(sql: string): void {
+    if (!this.sqlite) {
+      throw new Error("connect() must be called before exec()");
+    }
+    this.sqlite.exec(sql);
+  }
+
   async close(): Promise<void> {
     if (this.sqlite) {
       this.sqlite.close();