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>

apps/aelis-backend/src/lib/crypto.ts

@@ -0,0 +1,60 @@
+import { createCipheriv, createDecipheriv, randomBytes } from "node:crypto"
+
+const ALGORITHM = "aes-256-gcm"
+const IV_LENGTH = 12
+const AUTH_TAG_LENGTH = 16
+
+/**
+ * AES-256-GCM encryption for credential storage.
+ *
+ * Caches the parsed key on construction to avoid repeated
+ * env reads and Buffer allocations.
+ */
+export class CredentialEncryptor {
+	private readonly key: Buffer
+
+	constructor(base64Key: string) {
+		const key = Buffer.from(base64Key, "base64")
+		if (key.length !== 32) {
+			throw new Error(
+				`Encryption key must be 32 bytes (got ${key.length}). Generate with: openssl rand -base64 32`,
+			)
+		}
+		this.key = key
+	}
+
+	/**
+	 * Encrypts plaintext using AES-256-GCM.
+	 *
+	 * Output format: [12-byte IV][ciphertext][16-byte auth tag]
+	 */
+	encrypt(plaintext: string): Buffer {
+		const iv = randomBytes(IV_LENGTH)
+		const cipher = createCipheriv(ALGORITHM, this.key, iv)
+
+		const encrypted = Buffer.concat([cipher.update(plaintext, "utf8"), cipher.final()])
+		const authTag = cipher.getAuthTag()
+
+		return Buffer.concat([iv, encrypted, authTag])
+	}
+
+	/**
+	 * Decrypts a buffer produced by `encrypt`.
+	 *
+	 * Expects format: [12-byte IV][ciphertext][16-byte auth tag]
+	 */
+	decrypt(data: Buffer): string {
+		if (data.length < IV_LENGTH + AUTH_TAG_LENGTH) {
+			throw new Error("Encrypted data is too short")
+		}
+
+		const iv = data.subarray(0, IV_LENGTH)
+		const authTag = data.subarray(data.length - AUTH_TAG_LENGTH)
+		const ciphertext = data.subarray(IV_LENGTH, data.length - AUTH_TAG_LENGTH)
+
+		const decipher = createDecipheriv(ALGORITHM, this.key, iv)
+		decipher.setAuthTag(authTag)
+
+		return Buffer.concat([decipher.update(ciphertext), decipher.final()]).toString("utf8")
+	}
+}