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/network.py

@@ -15,6 +15,10 @@ from ipaddress import IPv4Address, IPv4Interface, IPv4Network
 
 import docker
 
+from decnet.logging import get_logger
+
+log = get_logger("network")
+
 MACVLAN_NETWORK_NAME = "decnet_lan"
 HOST_MACVLAN_IFACE = "decnet_macvlan0"
 HOST_IPVLAN_IFACE = "decnet_ipvlan0"
@@ -491,9 +495,12 @@ def get_container_veth(container_name: str) -> str:
         check=False,
     )
     if result.returncode != 0:
-        raise LookupError(
-            f"container {container_name!r} not reachable: {result.stderr.strip()}"
+        log.warning(
+            "get_container_veth: docker exec failed for container %r: %s",
+            container_name,
+            result.stderr.strip(),
         )
+        raise LookupError(f"container {container_name!r} not reachable")
     peer_index = result.stdout.strip()
     links = _run(["ip", "link", "show"])
     for line in links.stdout.splitlines():