feat: add cancel, pause, and resume flow control for scans

Users running large scans can now pause (keep partial results, resume
later), cancel (stop permanently, partial results preserved), or resume
a paused scan which races through cache hits before continuing.

Backend:
- Extend scans.status CHECK to include 'paused' and 'cancelled'
- Add _migrate_add_pause_cancel_status() table-recreation migration
- scan_processor: _running_tasks/_cancel_reasons registries,
  cancel_scan_task/pause_scan_task/stop_scan_task helpers,
  CancelledError handler in process_scan(), start_resume_processor()
- api_server: POST /scans/{id}/pause|cancel|resume endpoints with
  rate limits (30/min pause+cancel, 10/min resume); list_scans now
  accepts paused/cancelled as status filter values

Frontend:
- Scan.status type extended with 'paused' | 'cancelled'
- scanApi.pause/cancel/resume added
- StatusChip: amber PauseCircle chip for paused, grey Ban for cancelled
- ScanDetails: context-aware action row with inline-confirm for
  Pause and Cancel; Resume button for paused scans

Tests: 129 total (58 new) across test_scan_control.py,
test_scan_processor_control.py, and additions to existing suites

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

flight-comparator/database/init_db.py

@@ -199,6 +199,108 @@ def _migrate_add_scheduled_scan_id_to_scans(conn, verbose=True):
         print("   ✅ Migration complete: scheduled_scan_id column added to scans")
 
 
+def _migrate_add_timing_columns_to_scans(conn, verbose=True):
+    """
+    Migration: add started_at and completed_at columns to the scans table.
+
+    started_at  — set when status transitions to 'running'
+    completed_at — set when status transitions to 'completed' or 'failed'
+    Both are nullable so existing rows are unaffected.
+    """
+    cursor = conn.execute("PRAGMA table_info(scans)")
+    columns = [row[1] for row in cursor.fetchall()]
+    if not columns:
+        return  # Fresh install: scans table doesn't exist yet — schema will create the columns
+    if 'started_at' in columns and 'completed_at' in columns:
+        return  # Already migrated
+
+    if verbose:
+        print("   🔄 Migrating scans table: adding started_at and completed_at columns...")
+
+    if 'started_at' not in columns:
+        conn.execute("ALTER TABLE scans ADD COLUMN started_at TIMESTAMP")
+    if 'completed_at' not in columns:
+        conn.execute("ALTER TABLE scans ADD COLUMN completed_at TIMESTAMP")
+    conn.commit()
+
+    if verbose:
+        print("   ✅ Migration complete: started_at and completed_at columns added to scans")
+
+
+def _migrate_add_pause_cancel_status(conn, verbose=True):
+    """
+    Migration: Extend status CHECK constraint to include 'paused' and 'cancelled'.
+
+    Needed for cancel/pause/resume scan flow control feature.
+    Uses the same table-recreation pattern as _migrate_relax_country_constraint
+    because SQLite doesn't support modifying CHECK constraints in-place.
+    """
+    cursor = conn.execute(
+        "SELECT sql FROM sqlite_master WHERE type='table' AND name='scans'"
+    )
+    row = cursor.fetchone()
+    if not row or 'paused' in row[0]:
+        return  # Table doesn't exist yet (fresh install) or already migrated
+
+    if verbose:
+        print("   🔄 Migrating scans table: adding 'paused' and 'cancelled' status values...")
+
+    # SQLite doesn't support ALTER TABLE MODIFY COLUMN, so recreate the table.
+    # Use PRAGMA foreign_keys = OFF to avoid FK errors during the swap.
+    conn.execute("PRAGMA foreign_keys = OFF")
+    # Drop triggers that reference scans (they are recreated by executescript below).
+    conn.execute("DROP TRIGGER IF EXISTS update_scans_timestamp")
+    conn.execute("DROP TRIGGER IF EXISTS update_scan_flight_count_insert")
+    conn.execute("DROP TRIGGER IF EXISTS update_scan_flight_count_update")
+    conn.execute("DROP TRIGGER IF EXISTS update_scan_flight_count_delete")
+    conn.execute("""
+        CREATE TABLE scans_new (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            origin TEXT NOT NULL CHECK(length(origin) = 3),
+            country TEXT NOT NULL CHECK(length(country) >= 2),
+            start_date TEXT NOT NULL,
+            end_date TEXT NOT NULL,
+            created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+            updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+            started_at TIMESTAMP,
+            completed_at TIMESTAMP,
+            status TEXT NOT NULL DEFAULT 'pending'
+                CHECK(status IN ('pending', 'running', 'completed', 'failed', 'cancelled', 'paused')),
+            total_routes INTEGER NOT NULL DEFAULT 0 CHECK(total_routes >= 0),
+            routes_scanned INTEGER NOT NULL DEFAULT 0 CHECK(routes_scanned >= 0),
+            total_flights INTEGER NOT NULL DEFAULT 0 CHECK(total_flights >= 0),
+            error_message TEXT,
+            seat_class TEXT DEFAULT 'economy',
+            adults INTEGER DEFAULT 1 CHECK(adults > 0 AND adults <= 9),
+            scheduled_scan_id INTEGER,
+            CHECK(end_date >= start_date),
+            CHECK(routes_scanned <= total_routes OR total_routes = 0)
+        )
+    """)
+    # Use named columns to handle different column orderings (ALTER TABLE vs fresh schema).
+    conn.execute("""
+        INSERT INTO scans_new (
+            id, origin, country, start_date, end_date,
+            created_at, updated_at, started_at, completed_at,
+            status, total_routes, routes_scanned, total_flights,
+            error_message, seat_class, adults, scheduled_scan_id
+        )
+        SELECT
+            id, origin, country, start_date, end_date,
+            created_at, updated_at, started_at, completed_at,
+            status, total_routes, routes_scanned, total_flights,
+            error_message, seat_class, adults, scheduled_scan_id
+        FROM scans
+    """)
+    conn.execute("DROP TABLE scans")
+    conn.execute("ALTER TABLE scans_new RENAME TO scans")
+    conn.execute("PRAGMA foreign_keys = ON")
+    conn.commit()
+
+    if verbose:
+        print("   ✅ Migration complete: status now accepts 'paused' and 'cancelled'")
+
+
 def initialize_database(db_path=None, verbose=True):
     """
     Initialize or migrate the database.
@@ -245,6 +347,8 @@ def initialize_database(db_path=None, verbose=True):
     _migrate_relax_country_constraint(conn, verbose)
     _migrate_add_routes_unique_index(conn, verbose)
     _migrate_add_scheduled_scan_id_to_scans(conn, verbose)
+    _migrate_add_timing_columns_to_scans(conn, verbose)
+    _migrate_add_pause_cancel_status(conn, verbose)
 
     # Load and execute schema
     schema_sql = load_schema()