Implement 14 power-user feature requests for field deployment

Critical:
- FR-01: Chain verification now supports key rotation via signed rotation
  records (soosef/key-rotation-v1 content type). Old single-signer
  invariant replaced with authorized-signers set.
- FR-02: Carrier images stripped of EXIF metadata by default before
  steganographic encoding (strip_metadata=True). Prevents source
  location/device leakage.

High priority:
- FR-03: Session timeout (default 15min) + secure cookie flags
  (HttpOnly, SameSite=Strict, Secure when HTTPS)
- FR-04: CSRF protection via Flask-WTF on all POST forms. Killswitch
  now requires password re-authentication.
- FR-05: Collaborator trust store — trust_key(), get_trusted_keys(),
  resolve_attestor_name(), untrust_key() in KeystoreManager.
- FR-06: Production WSGI server (Waitress) by default, Flask dev
  server only with --debug flag.
- FR-07: Dead man's switch sends warning during grace period via
  local file + optional webhook before auto-purge.

Medium:
- FR-08: Geofence get_current_location() via gpsd for --here support.
- FR-09: Batch attestation endpoint (/attest/batch) with SHA-256
  dedup and per-file status reporting.
- FR-10: Key backup tracking with last_backup_info() and
  is_backup_overdue() + backup_reminder_days config.
- FR-11: Verification receipts signed with instance Ed25519 key
  (schema_version bumped to 2).
- FR-12: Login rate limiting with configurable lockout (5 attempts,
  15 min default).

Nice-to-have:
- FR-13: Unified `soosef status` pre-flight command showing identity,
  channel key, deadman, geofence, chain, and backup status.
- FR-14: `soosef chain export` produces ZIP with JSON manifest,
  public key, and raw chain.bin for legal discovery.

Tests: 157 passed, 1 skipped, 1 pre-existing flaky test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontends/web/templates/login.html

@@ -12,6 +12,7 @@
             </div>
             <div class="card-body">
                 <form method="POST" action="{{ url_for('login') }}">
+              <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
                     <div class="mb-3">
                         <label class="form-label">
                             <i class="bi bi-person me-1"></i> Username