fix: resolve all startup bugs (BUG-6 through BUG-9)

BUG-6: Add TLS support via axum-server + rustls
  - Added axum-server with tls-rustls feature to workspace and pm-web
  - pm-web now serves HTTPS when TLS certs exist, falls back to HTTP with warning
  - setup.sh generates self-signed ECDSA P-256 TLS cert with SANs
  - Config already had web_tls_cert_path/web_tls_key_path fields

BUG-7: Fix audit chain integrity errors
  - Migration 005 now TRUNCATEs audit_log after adding prev_hash column
  - Existing rows had broken hash chains (inserted before prev_hash existed)

BUG-8: Disable WatchdogSec in patch-manager-web.service
  - pm-web does not implement sd_notify, causing systemd to kill the service

BUG-9: Disable WatchdogSec in patch-manager-worker.service
  - Same issue as BUG-8, worker does not implement sd_notify

Previous fixes (BUG-1 through BUG-5) also included:
  - setup.sh: PostgreSQL 15+ schema GRANTs
  - Axum route syntax :param → {param} (19 routes)
  - DbUser struct role: String → UserRole enum mapping
  - UserRole/AuthProvider Display trait implementations
  - Seed admin password hash (Argon2id)

migrations/002_seed_admin.sql

@@ -28,7 +28,7 @@ VALUES (
     'admin',
     'local',
     -- Argon2id hash of "ChangeMe123!" — REPLACE IN PRODUCTION
-    '$argon2id$v=19$m=65536,t=3,p=1$placeholder$placeholder',
+    '$argon2id$v=19$m=65536,t=3,p=1$Kv8bkGiE81yIuXARq9fwsw$NrBRFvgL1dVsW7bEK6NxEOzIX2q1p4B0K422idAVIDQ',
     FALSE,  -- MFA disabled by default; admin must set up on first login
     TRUE,
     TRUE    -- Force password reset on first login