fix(security): close LOW ASVS findings — env bypass, SSE/deployment authz, CN fail-close, password byte-limit, exception leaks, BUG-12..16

Auth/session (V2.1.7, V4.1.5, V4.1.6, V2.1.4/V2.1.5):
- env secret validation no longer bypassed by attacker-injectable PYTEST* env;
  gated on explicit DECNET_TESTING=1 (set only in conftest).
- must_change_password now enforced on the SSE header-JWT path, not just ticket mint.
- GET /system/deployment-mode requires viewer auth (was leaking role + topology size).
- CreateUser/ResetUser passwords min_length=12; passwords >72 bytes rejected
  explicitly instead of bcrypt silently truncating.

Swarm ingestion (V9.1.3, BUG-16):
- Log listener hard-rejects peers with unparseable/empty cert CN (fail closed,
  ingests nothing) instead of tagging 'unknown'.
- Shutdown handlers no longer swallow real errors (narrowed to CancelledError).

Info leakage (V7.1.2, V14.1.2):
- Exception text sanitized on swarm-update, health, tarpit, realism, file-drop,
  blank-topology endpoints (raw tc/docker stderr, DB/Docker errors logged
  server-side, generic detail returned). pyproject license corrected to AGPL-3.0.

Correctness (BUG-12..16):
- BUG-12 atomic credential upsert (UNIQUE constraint + IntegrityError retry,
  consistent principal_key canonicalization).
- BUG-13 rule-tail watermark uses >= with seen-id dedup (no same-second drop).
- BUG-14 worker wake cleared before wait (no lost wake during tick).
- BUG-15 intel gather tolerates an unexpected provider raise.
- BUG-16 see above.

Already-closed (verified, no change): V2.1.6, V5.1.3, V9.1.2. Accept-risk +
documented: V2.1.8 cache window, V3.1.3 idle timeout. Tests added for every fix;
unanimous adversarial review after two refute-fix rounds.

decnet/web/router/system/api_deployment_mode.py

@@ -13,7 +13,7 @@ from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 
 from decnet.web.db.repository import BaseRepository
-from decnet.web.dependencies import get_repo
+from decnet.web.dependencies import get_repo, require_viewer
 
 router = APIRouter()
 
@@ -24,9 +24,15 @@ class DeploymentModeResponse(BaseModel):
     swarm_host_count: int
 
 
+# Auth-gated (V4.1.6): the response leaks host role + enrolled-worker count,
+# which is recon-useful to an unauthenticated attacker. The dashboard only ever
+# calls this from inside the post-login app shell (App.tsx gates the whole app
+# behind a valid token), so there is no pre-auth UI-mode use case to preserve —
+# gate the entire endpoint behind require_viewer rather than splitting it.
 @router.get("/deployment-mode", response_model=DeploymentModeResponse)
 async def get_deployment_mode(
     repo: BaseRepository = Depends(get_repo),
+    _user: dict = Depends(require_viewer),
 ) -> DeploymentModeResponse:
     role = os.environ.get("DECNET_MODE", "master").lower()
     hosts = 0