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/realism/api_llm.py

@@ -145,23 +145,26 @@ async def put_llm_config(
         try:
             from decnet.web.db.secrets import encrypt_secret
             merged["api_key_ciphertext"] = encrypt_secret(str(api_key_raw))
-        except RuntimeError as exc:
+        except RuntimeError:
+            log.exception("api.realism.put_llm: secret encryption unavailable")
             raise HTTPException(
                 status_code=500,
-                detail=f"Secret encryption unavailable: {exc}",
-            ) from exc
+                detail="Secret encryption unavailable; check server configuration.",
+            ) from None
 
     try:
         cfg = LLMConfig(**merged)
     except Exception as exc:
-        raise HTTPException(status_code=400, detail=str(exc)) from exc
+        log.warning("api.realism.put_llm: LLMConfig validation failed: %s", exc)
+        raise HTTPException(status_code=400, detail="Invalid LLM configuration payload.") from exc
 
     try:
         llm_config.apply(cfg)
-    except Exception as exc:
+    except Exception:
+        log.exception("api.realism.put_llm: backend init failed")
         raise HTTPException(
-            status_code=400, detail=f"Backend init failed: {exc}"
-        ) from exc
+            status_code=400, detail="Backend init failed; check provider/model settings."
+        ) from None
 
     await repo.set_realism_config(_CONFIG_KEY, json.dumps(merged))
     _hydrated = True