fix(security): close MEDIUM ASVS findings — JWT pinning, SSE tickets, SSRF, mTLS pin, rate limits + correctness bugs

Auth (V2.1.1/V3.1.2, V2.1.3, V3.1.1):
- Pin JWT iss/aud/typ at mint and require+verify them at decode; revocation
  (jti denylist + tokens_valid_from) still enforced.
- Change-password now requires min_length=12.
- SSE auth moves off JWT-in-URL to a single-use 60s opaque ticket
  (POST /auth/sse-ticket); raw JWT in query no longer authenticates a stream.
  Removed dead fail-open get_stream_user helper.

Egress (V5.1.1, V9.1.1/V14.1.3):
- Webhook delivery + CRUD reject SSRF destinations (private/loopback/link-local/
  metadata, IPv4-mapped, multi-A-record) via resolved-IP validation, pin to the
  vetted IP, and never auto-follow redirects. Opt-out via DECNET_WEBHOOK_ALLOW_PRIVATE.
- UpdaterClient pins the worker leaf cert SHA-256 against the stored per-host
  fingerprint (fail closed on missing/mismatch); DECNET_VERIFY_HOSTNAME now
  defaults True.

Hardening (V13.1.3, V4.1.4, V13.1.2):
- Rate-limit change-password (5/min), enroll-bundle (10/min), webhook-create
  (20/min), host-delete (20/min) via the existing slowapi limiter.
- Correct false 'global auth middleware' comment; document enroll-bundle proxy
  trust.

Correctness (BUG-7..11):
- BUG-7 unbound bus in finally; BUG-8 apply_ceiling clamps to min(base,ceiling);
  BUG-9 commit before emit; BUG-10 multi-actor rearm for sub-threshold identities;
  BUG-11 normalize naive timestamps to UTC.

Already-closed (no change): V14.1.1, V2.1.2/V3.1.3, V5.1.2. Tests added for
every fix; unanimous adversarial review.

decnet/env.py

@@ -204,13 +204,26 @@ _cors_raw: str = os.environ.get("DECNET_CORS_ORIGINS", _cors_default)
 DECNET_CORS_ORIGINS: list[str] = [o.strip() for o in _cors_raw.split(",") if o.strip()]
 
 
-# Master→worker mTLS hostname verification. Off by default because legacy
-# enrollments were issued certs with operator-supplied SAN lists that may
-# not match the URL the master uses to connect; set to "true" on a fresh
-# production deploy where you control enrollment to get TLS hostname checks
-# on top of CA + fingerprint pinning.
+# Master→worker mTLS hostname verification. ON by default — the worker's
+# cert SAN must match the address the master connects to, on top of CA +
+# SHA-256 fingerprint pinning. Operators with legacy enrollments whose
+# operator-supplied SAN lists don't match the connect URL can opt OUT
+# explicitly with DECNET_VERIFY_HOSTNAME=false, but that is a downgrade:
+# it drops SAN binding and leans entirely on CA + per-host pinning.
 DECNET_VERIFY_HOSTNAME: bool = (
-    os.environ.get("DECNET_VERIFY_HOSTNAME", "false").lower() == "true"
+    os.environ.get("DECNET_VERIFY_HOSTNAME", "true").lower() == "true"
+)
+
+
+# Webhook egress SSRF guard. By default DECNET refuses to deliver a webhook
+# to a private (RFC1918), loopback, link-local (incl. 169.254.169.254 cloud
+# metadata), unspecified, reserved, or multicast destination, and rejects
+# such URLs at registration time. Operators who genuinely need to target an
+# internal receiver (e.g. an on-box SIEM) opt IN explicitly by setting
+# DECNET_WEBHOOK_ALLOW_PRIVATE=true. Fails closed: anything other than the
+# literal "true" leaves the guard fully enabled.
+DECNET_WEBHOOK_ALLOW_PRIVATE: bool = (
+    os.environ.get("DECNET_WEBHOOK_ALLOW_PRIVATE", "false").lower() == "true"
 )