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/sql-timestamp.ts

@@ -0,0 +1,20 @@
+/**
+ * Timestamp columns are stored as text defaulting to `CURRENT_TIMESTAMP`, which
+ * every supported engine writes as `YYYY-MM-DD HH:MM:SS` in UTC. That format
+ * sorts lexicographically in time order, so retention cutoffs can be plain
+ * string comparisons.
+ *
+ * Computing the cutoff here rather than with `datetime('now', ?)` keeps the
+ * queries free of engine-specific date functions.
+ */
+export function sqlTimestampDaysAgo(
+  days: number,
+  now: Date = new Date(),
+): string {
+  const cutoff = new Date(now.getTime() - days * 24 * 60 * 60 * 1000);
+  return formatSqlTimestamp(cutoff);
+}
+
+export function formatSqlTimestamp(date: Date): string {
+  return date.toISOString().slice(0, 19).replace("T", " ");
+}