Add tamper-evidence: package-DB integrity, self-integrity, dead-man's switch

Answers two hard questions: "what guards the hashes FIM trusts?" and "how do we
make the sensor itself hard to silently disable?" — within the honest limit that
a root attacker who shares your privileges can't be fully stopped on-box, only
made loud.

- pkgdb.py: guards the package DB that `pacman -Qkk` trusts. Anchors a
  fingerprint of /var/lib/pacman/local (refreshed ONLY by the pacman hook) and
  cross-checks pacman.log; a DB change with no logged transaction is flagged
  pkgdb_tamper (CRITICAL, sid 100021) — catches an attacker rewriting a stored
  checksum to mask a modified binary. Verified end-to-end against a simulated
  hash-overwrite.
- selfprotect.py: Sentinel's own binaries/config/units/hook are always in the
  FIM watch set (self-integrity); a heartbeat is written each loop; an external
  `watchdog` command polls a remote dashboard and pushes if the sensor goes
  silent or unreachable — silence becomes the alarm.
- daemon: heartbeat + slow-cadence pkgdb check; DB anchor re-anchored in
  build_fim_baseline so the pacman hook refreshes it after each transaction
- web: /api/status now reports heartbeat_age/stale; dashboard shows it
- cli: pkgdb-check, watchdog (--url/--token/--max-age, bypasses min-severity)
- config: pkgdb_verify/_interval, heartbeat_max_age
- README: threat model (trust-anchor problem) + hardening layers (immutability,
  signed-package + external anchor); reframes "anti-rootkit" as tamper-evidence
- tests: +8 (DB fingerprint, anchor/transaction logic, pacman.log parse,
  heartbeat + watchdog verdicts). 80/80 pass

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

enodia_sentinel/web.py

@@ -141,6 +141,8 @@ def daemon_status(cfg: Config) -> dict:
         if "eBPF exec monitor:" in line:
             ebpf = line.split("eBPF exec monitor:", 1)[1].strip()
             break
+    from .selfprotect import heartbeat_age
+    age = heartbeat_age(cfg)
     return {
         "version": __version__,
         "running": pid_alive,
@@ -149,6 +151,8 @@ def daemon_status(cfg: Config) -> dict:
         "last_alert": alerts[0]["time"] if alerts else None,
         "ebpf": ebpf,
         "host": os.uname().nodename,
+        "heartbeat_age": age,
+        "heartbeat_stale": (age is not None and age > cfg.heartbeat_max_age),
     }