security: JWT aud/iss validation, token blacklist, encryption key crash-early

C-2 (CRITICAL): JWT decode_token now validates aud, iss, iat, jti claims.
  Tokens without these claims are rejected. Audience 'ss-tools-api' and
  issuer 'ss-tools' prevent cross-service token reuse.

H-2 (HIGH): Server-side JWT revocation via token_blacklist table.
  - New TokenBlacklist model (SHA-256 hash only, never raw token)
  - blacklist_token() on logout — revokes current session
  - is_token_blacklisted() check in get_current_user
  - Expired entries auto-pruned via _prune_blacklist()

H-3 (HIGH): Encryption key auto-generation removed — crash-early instead.
  Previously, ensure_encryption_key() would auto-generate a Fernet key
  on first run and write it to .env. This made encrypted data unrecoverable
  when the key changed between deployments. Now raises RuntimeError with
  a clear command to generate the key explicitly.

Also: update orthogonal security test for crash-early behavior.

backend/tests/test_security_orthogonal.py

@@ -234,31 +234,27 @@ class TestEncryptionKeyLifecycle:
             assert result == expected
     # #endregion test_ensure_encryption_key_from_env
 
-    # #region test_ensure_encryption_key_creates_file [C:2] [TYPE Function]
-    # @BRIEF When neither env nor .env has a key, generate one and persist to .env.
-    def test_ensure_encryption_key_creates_file(self):
+    # #region test_ensure_encryption_key_missing_raises [C:2] [TYPE Function]
+    # @BRIEF When neither env nor .env has a key, crash-early with RuntimeError.
+    # @TEST_EDGE: missing_encryption_key -> RuntimeError with clear message
+    def test_ensure_encryption_key_missing_raises(self):
         from src.core.encryption_key import ensure_encryption_key
 
         with tempfile.NamedTemporaryFile(mode="w", suffix=".env", delete=False) as f:
+            f.write("# empty .env\n")
             env_path = Path(f.name)
 
         try:
-            # Remove any existing ENCRYPTION_KEY from env
             old = os.environ.pop("ENCRYPTION_KEY", None)
             try:
-                result = ensure_encryption_key(env_path)
-                assert len(result) > 0
-
-                # Key should be in the file
-                content = env_path.read_text()
-                assert "ENCRYPTION_KEY=" in content
-                assert result in content
+                with pytest.raises(RuntimeError, match="ENCRYPTION_KEY"):
+                    ensure_encryption_key(env_path)
             finally:
                 if old is not None:
                     os.environ["ENCRYPTION_KEY"] = old
         finally:
             env_path.unlink(missing_ok=True)
-    # #endregion test_ensure_encryption_key_creates_file
+    # #endregion test_ensure_encryption_key_missing_raises
 
     # #region test_ensure_encryption_key_loads_from_file [C:2] [TYPE Function]
     # @BRIEF When env is empty but .env file has the key, load it.