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/database/repositories/database-context.ts

@@ -1,9 +1,22 @@
 import type { BetterSQLite3Database } from "drizzle-orm/better-sqlite3";
-import type { Database as BetterSqliteDatabase } from "better-sqlite3";
 import type * as schema from "../db/schema.js";
 
+/**
+ * Engines the repository layer can run against. SQLite is the only one wired up
+ * today; the alias exists so that adding another is a change in one place
+ * rather than a hunt for string literals.
+ */
+export type DatabaseDialect = "sqlite";
+
+/**
+ * What a repository is allowed to touch.
+ *
+ * Deliberately drizzle-only: with no raw driver handle here, no repository can
+ * reach for engine-specific SQL. Retention queries that previously needed
+ * `datetime('now', ?)` compute their cutoff in JS instead — see
+ * ./sql-timestamp.ts.
+ */
 export interface DatabaseContext {
-  dialect: "sqlite";
+  dialect: DatabaseDialect;
   drizzle: BetterSQLite3Database<typeof schema>;
-  sqlite?: BetterSqliteDatabase;
 }