fix: stop gitignore from matching lib/secrets/ application code

The .gitignore rule `secrets/` matched any directory named secrets/
anywhere in the tree, including the new lib/secrets/ application code
that holds the AES-GCM helpers. Production build failed with
"Module not found: Can't resolve '@/lib/secrets/crypto'". Anchor the
ignore to the repo root with `/secrets/` so only the top-level
credentials directory is excluded.

.gitignore

@@ -27,6 +27,7 @@ playwright-report/
 supabase/.branches/
 supabase/.temp/
 
-# Production deployment secrets (never commit)
-secrets/
+# Production deployment secrets (never commit). Anchored to repo root so it
+# doesn't also match lib/secrets/ (which is application code, not credentials).
+/secrets/
 *.pem

lib/secrets/crypto.ts

@@ -0,0 +1,56 @@
+// AES-256-GCM encryption for the Secrets vault.
+//
+// Key handling:
+//   The master key comes from process.env.SECRETS_ENCRYPTION_KEY (64 hex chars).
+//   It MUST NOT be reachable from the browser; this module is server-only.
+//
+// Storage layout (encryption_version = 1):
+//   ciphertext field bytes = iv (12) || ciphertext (N) || authTag (16)
+//
+// A per-secret IV is generated on every encrypt. Decryption rejects any payload
+// shorter than 28 bytes (12 IV + 16 tag) before touching the crypto primitive.
+
+import crypto from 'node:crypto'
+
+const VERSION = 1
+const IV_LEN = 12
+const TAG_LEN = 16
+
+function readKey(): Buffer {
+  const hex = process.env.SECRETS_ENCRYPTION_KEY
+  if (!hex) {
+    throw new Error('SECRETS_ENCRYPTION_KEY is not set. Generate with `openssl rand -hex 32` and add to your environment.')
+  }
+  if (!/^[0-9a-fA-F]{64}$/.test(hex)) {
+    throw new Error('SECRETS_ENCRYPTION_KEY must be exactly 64 hex characters (32 bytes).')
+  }
+  return Buffer.from(hex, 'hex')
+}
+
+export function encryptionAvailable(): boolean {
+  const hex = process.env.SECRETS_ENCRYPTION_KEY
+  return !!hex && /^[0-9a-fA-F]{64}$/.test(hex)
+}
+
+export function encryptSecret(plaintext: string): { ciphertext: Buffer; version: number } {
+  const key = readKey()
+  const iv = crypto.randomBytes(IV_LEN)
+  const cipher = crypto.createCipheriv('aes-256-gcm', key, iv)
+  const ct = Buffer.concat([cipher.update(plaintext, 'utf8'), cipher.final()])
+  const tag = cipher.getAuthTag()
+  return { ciphertext: Buffer.concat([iv, ct, tag]), version: VERSION }
+}
+
+export function decryptSecret(buf: Buffer | Uint8Array | null | undefined, version: number): string {
+  if (!buf) throw new Error('No ciphertext')
+  const data = Buffer.isBuffer(buf) ? buf : Buffer.from(buf)
+  if (version !== VERSION) throw new Error(`Unknown ciphertext version ${version}`)
+  if (data.length < IV_LEN + TAG_LEN + 1) throw new Error('Ciphertext too short')
+  const iv = data.subarray(0, IV_LEN)
+  const tag = data.subarray(data.length - TAG_LEN)
+  const ct = data.subarray(IV_LEN, data.length - TAG_LEN)
+  const key = readKey()
+  const decipher = crypto.createDecipheriv('aes-256-gcm', key, iv)
+  decipher.setAuthTag(tag)
+  return Buffer.concat([decipher.update(ct), decipher.final()]).toString('utf8')
+}