fix: full security audit remediation — P0/P1/P2/P3 fixes + 1020 passing tests

P0 — Broken functionality:
- Fix 12+ endpoints with wrong manager method signatures (email/calendar/file/routing)
- Fix email_manager.delete_email_user() missing domain arg
- Fix cell-link DNS forwarding wiped on every peer change (generate_corefile now
  accepts cell_links param; add/remove_cell_dns_forward no longer clobber the file)
- Fix Flask SECRET_KEY regenerating on every restart (persisted to DATA_DIR)
- Fix _next_peer_ip exhaustion returning 500 instead of 409
- Fix ConfigManager Caddyfile path (/app/config-caddy/)
- Fix UI double-add and wrong-key peer bugs in Peers.jsx / WireGuard.jsx
- Remove hardcoded credentials from Dashboard.jsx

P1 — Security:
- CSRF token validation on all POST/PUT/DELETE/PATCH to /api/* (double-submit pattern)
- enforce_auth: 503 only when users file readable but empty; never bypass on IOError
- WireGuard add_cell_peer: validate pubkey, name, endpoint against strict regexes
- DNS add_cell_dns_forward: validate IP and domain; reject injection chars
- DNS zone write: realpath containment + record content validation
- iptables comment /32 suffix prevents substring match deleting wrong peer rules
- is_local_request() trusts only loopback + 172.16.0.0/12 (Docker bridge)
- POST /api/containers: volume allow-list prevents arbitrary host mounts
- file_manager: bcrypt ($2b→$2y) for WebDAV; realpath containment in delete_user
- email/calendar: stop persisting plaintext passwords in user records
- routing_manager: validate IPs, networks, and interface names
- peer_registry: write peers.json at mode 0o600
- vault_manager: Fernet key file at mode 0o600
- CORS: lock down to explicit origin list
- domain/cell_name validation: reject newline, brace, semicolon injection chars

P2 — Architecture:
- Peer add: rollback registry entry if firewall rules fail post-add
- restart_service(): base class now calls _restart_container(); email and calendar
  managers call cell-mail / cell-radicale respectively
- email/calendar managers sync user list (no passwords) to cell_config.json
- Pending-restart flag cleared only after helper subprocess exits with code 0
- docker-compose.yml: add config-caddy volume to API container

P3 — Tests (854 → 1020):
- Fill test_email_endpoints.py, test_calendar_endpoints.py,
  test_network_endpoints.py, test_routing_endpoints.py
- New: test_peer_management_update.py, test_peer_management_edge_cases.py,
  test_input_validation.py, test_enforce_auth_configured.py,
  test_cell_link_dns.py, test_logs_endpoints.py, test_cells_endpoints.py,
  test_is_local_request_per_endpoint.py, test_caddy_routing.py
- E2E conftest: skip WireGuard suite when wg-quick absent
- Update existing tests to match fixed signatures and comment formats

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_app_misc.py

@@ -113,8 +113,11 @@ class TestAppMisc(unittest.TestCase):
             self.assertFalse(app_module.is_local_request())
 
     def test_is_local_request_private_ip(self):
+        # 192.168.x.x (LAN) is no longer trusted — only Docker bridge (172.16.0.0/12)
+        # and loopback are trusted. The API is bound to 127.0.0.1:3000 and only
+        # reachable via Caddy (172.20.x.x), so LAN IPs never reach it directly.
         with patch('app.request', new=self._req('192.168.1.5')):
-            self.assertTrue(app_module.is_local_request())
+            self.assertFalse(app_module.is_local_request())
 
     def test_is_local_request_xff_spoof_rejected(self):
         # Client sends X-Forwarded-For: 127.0.0.1 but actual IP is public
@@ -123,8 +126,14 @@ class TestAppMisc(unittest.TestCase):
             self.assertFalse(app_module.is_local_request())
 
     def test_is_local_request_xff_last_entry_local(self):
-        # Caddy appends the real client IP; last entry is local → allow
+        # 192.168.x.x is no longer in the trusted range — only Docker bridge
+        # (172.16.0.0/12) and loopback are trusted now.
         with patch('app.request', new=self._req('8.8.8.8', xff='8.8.8.8, 192.168.1.10')):
+            self.assertFalse(app_module.is_local_request())
+
+    def test_is_local_request_xff_docker_bridge(self):
+        # Docker bridge IPs (172.16.0.0/12) ARE trusted — Caddy uses this range
+        with patch('app.request', new=self._req('8.8.8.8', xff='8.8.8.8, 172.20.0.2')):
             self.assertTrue(app_module.is_local_request())
 
     def test_is_local_request_xff_single_public_rejected(self):