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/schema.ts

@@ -0,0 +1,62 @@
+import {
+	boolean,
+	customType,
+	index,
+	jsonb,
+	pgTable,
+	text,
+	timestamp,
+	unique,
+	uuid,
+} from "drizzle-orm/pg-core"
+
+// ---------------------------------------------------------------------------
+// Better Auth core tables
+// Re-exported from CLI-generated schema.
+// Regenerate with: bunx --bun auth@latest generate --config auth.ts --output src/db/auth-schema.ts
+// ---------------------------------------------------------------------------
+
+export {
+	user,
+	session,
+	account,
+	verification,
+	userRelations,
+	sessionRelations,
+	accountRelations,
+} from "./auth-schema.ts"
+
+import { user } from "./auth-schema.ts"
+
+// ---------------------------------------------------------------------------
+// AELIS — per-user source configuration
+// ---------------------------------------------------------------------------
+
+const bytea = customType<{ data: Buffer }>({
+	dataType() {
+		return "bytea"
+	},
+})
+
+export const userSources = pgTable(
+	"user_sources",
+	{
+		id: uuid("id").primaryKey().defaultRandom(),
+		userId: text("user_id")
+			.notNull()
+			.references(() => user.id, { onDelete: "cascade" }),
+		sourceId: text("source_id").notNull(),
+		enabled: boolean("enabled").notNull().default(true),
+		config: jsonb("config").default({}),
+		credentials: bytea("credentials"),
+		createdAt: timestamp("created_at").notNull().defaultNow(),
+		updatedAt: timestamp("updated_at")
+			.notNull()
+			.defaultNow()
+			.$onUpdate(() => new Date()),
+	},
+	(t) => [
+		unique("user_sources_user_id_source_id_unique").on(t.userId, t.sourceId),
+		index("user_sources_user_id_enabled_idx").on(t.userId, t.enabled),
+	],
+)