fix: LDAP conn leak, health timing info, security headers, link_stats size guard

- app.py: move conn.unbind() into finally block in api_avatar() so connection is always closed even if conn.search() throws
- app.py: remove elapsed-time strings from /health response (unauthenticated endpoint no longer leaks monitor timing)
- app.py: add after_request hook setting X-Content-Type-Options, X-Frame-Options, Referrer-Policy on all responses
- app.py: add 10 MB size guard on link_stats before JSON parse; log actual exception on parse failure
- app.py: wrap suppressions_page network_snapshot parse in try/except (same protection as index page)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -94,6 +94,14 @@ def _config() -> dict:
     return _cfg
 
 
+@app.after_request
+def add_security_headers(response):
+    response.headers.setdefault('X-Content-Type-Options', 'nosniff')
+    response.headers.setdefault('X-Frame-Options', 'DENY')
+    response.headers.setdefault('Referrer-Policy', 'strict-origin-when-cross-origin')
+    return response
+
+
 def _daemon_ok(last_check: str) -> bool:
     """Return True if monitor last checked within 20 minutes."""
     if not last_check or last_check == 'Never':
@@ -225,7 +233,11 @@ def suppressions_page():
     active = db.get_active_suppressions()
     history = db.get_suppression_history(limit=50)
     snapshot_raw = db.get_state('network_snapshot')
-    snapshot = json.loads(snapshot_raw) if snapshot_raw else {}
+    try:
+        snapshot = json.loads(snapshot_raw) if snapshot_raw else {}
+    except Exception as e:
+        logger.error(f'Failed to parse network_snapshot JSON: {e}')
+        snapshot = {}
     return render_template(
         'suppressions.html',
         user=user,
@@ -272,10 +284,13 @@ def api_network():
 def api_links():
     raw = db.get_state('link_stats')
     if raw:
+        if len(raw) > 10_000_000:
+            logger.error(f'link_stats exceeds 10 MB ({len(raw)} bytes); possible corruption')
+            return jsonify({'error': 'Invalid cached data'}), 503
         try:
             return jsonify(json.loads(raw))
-        except Exception:
-            logger.error('Failed to parse link_stats JSON')
+        except Exception as e:
+            logger.error(f'Failed to parse link_stats JSON: {e}')
     return jsonify({'hosts': {}, 'updated': None})
 
 
@@ -548,6 +563,7 @@ def api_avatar():
         return '', 404
 
     avatar_data = None
+    conn = None
     try:
         import ldap3
         server = ldap3.Server(ldap_cfg['host'], port=int(ldap_cfg.get('port', 3890)))
@@ -560,13 +576,18 @@ def api_avatar():
                     f'(uid={safe_uid})', attributes=['avatar'])
         if conn.entries and conn.entries[0]['avatar'].value:
             avatar_data = conn.entries[0]['avatar'].value
-        conn.unbind()
     except ImportError:
         logger.error('ldap3 not installed — run: pip install ldap3')
         return '', 404
     except Exception as e:
         logger.error(f'LDAP avatar lookup failed for {username}: {e}')
         return '', 404
+    finally:
+        if conn is not None:
+            try:
+                conn.unbind()
+            except Exception:
+                pass
 
     if not avatar_data or len(avatar_data) < 100:
         with open(sentinel, 'w'): pass
@@ -613,10 +634,10 @@ def health():
             ts = datetime.strptime(last_check, '%Y-%m-%d %H:%M:%S UTC').replace(tzinfo=timezone.utc)
             age_s = (datetime.now(timezone.utc) - ts).total_seconds()
             if age_s > 1200:
-                checks['monitor'] = f'stale ({int(age_s)}s since last check)'
+                checks['monitor'] = 'stale'
                 overall = 'degraded'
             else:
-                checks['monitor'] = f'ok ({int(age_s)}s ago)'
+                checks['monitor'] = 'ok'
         else:
             checks['monitor'] = 'no data yet'
     except Exception as e: