feat(backend): add DB persistence layer (#79)

* feat(backend): add DB persistence layer

Replace raw pg Pool with Drizzle ORM backed by Bun.sql.
Add per-user source configuration table (user_sources).
Migrate Better Auth to drizzle-adapter.
Add AES-256-GCM credential encryption.

Co-authored-by: Ona <no-reply@ona.com>

* fix(backend): set updatedAt explicitly in all mutations

onConflictDoUpdate bypasses Drizzle's $onUpdate hook.
Set updatedAt explicitly in all mutation methods.

Co-authored-by: Ona <no-reply@ona.com>

* fix(backend): add composite index on user_sources

Add (user_id, enabled) index for the enabled() query path.

Co-authored-by: Ona <no-reply@ona.com>

---------

Co-authored-by: Ona <no-reply@ona.com>

apps/aelis-backend/src/db/index.ts

@@ -0,0 +1,23 @@
+import { SQL } from "bun"
+import { drizzle, type BunSQLDatabase } from "drizzle-orm/bun-sql"
+
+import * as schema from "./schema.ts"
+
+export type Database = BunSQLDatabase<typeof schema>
+
+export interface DatabaseConnection {
+	db: Database
+	close: () => Promise<void>
+}
+
+export function createDatabase(url: string): DatabaseConnection {
+	if (!url) {
+		throw new Error("DATABASE_URL is required")
+	}
+
+	const client = new SQL({ url })
+	return {
+		db: drizzle({ client, schema }),
+		close: () => client.close(),
+	}
+}