feat: Phase 3 - AES encryption, SQL audit engine, audit logs (85 tests)

Phase 3.1: Instance password AES-256-GCM encryption
Phase 3.2: SQL audit rule engine (10 rules)
Phase 3.3: Auto-audit on ticket creation
Phase 3.4: Operation audit logs + GET /audit/logs
Tests: 44 -> 85 (41 new), Frontend: AuditLogsPage

backend/app/core/crypto.py

@@ -0,0 +1,45 @@
+"""AES-256-GCM encryption for instance passwords."""
+import base64
+import os
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
+from app.core.config import get_settings
+
+_NONCE_SIZE = 12  # 96-bit nonce for AES-GCM
+
+
+def _get_key() -> bytes:
+    """Derive a 32-byte AES key from settings.SECRET_KEY."""
+    settings = get_settings()
+    # Use first 32 bytes of utf-8 encoded secret key (padded if needed)
+    raw = settings.SECRET_KEY.encode("utf-8")
+    if len(raw) >= 32:
+        return raw[:32]
+    return raw.ljust(32, b"\x00")
+
+
+def encrypt_password(plain: str) -> str:
+    """Encrypt a password string, return base64-encoded ciphertext (nonce + ciphertext)."""
+    key = _get_key()
+    nonce = os.urandom(_NONCE_SIZE)
+    aesgcm = AESGCM(key)
+    ct = aesgcm.encrypt(nonce, plain.encode("utf-8"), None)
+    return base64.b64encode(nonce + ct).decode("ascii")
+
+
+def decrypt_password(encrypted: str) -> str:
+    """Decrypt a base64-encoded encrypted password back to plaintext.
+    If the value is not valid base64 (e.g., legacy plaintext), return as-is.
+    """
+    try:
+        key = _get_key()
+        raw = base64.b64decode(encrypted, validate=True)
+        if len(raw) <= _NONCE_SIZE:
+            return encrypted  # too short to be encrypted
+        nonce = raw[:_NONCE_SIZE]
+        ct = raw[_NONCE_SIZE:]
+        aesgcm = AESGCM(key)
+        return aesgcm.decrypt(nonce, ct, None).decode("utf-8")
+    except Exception:
+        # Not encrypted (legacy plaintext password) — return as-is
+        return encrypted