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/scan_processor.py

@@ -21,6 +21,34 @@ from searcher_v3 import search_multiple_routes
 
 logger = logging.getLogger(__name__)
 
+# ─────────────────────────────────────────────────────────────────────────────
+# Task registry — tracks running asyncio tasks so they can be cancelled.
+# ─────────────────────────────────────────────────────────────────────────────
+
+_running_tasks: dict[int, asyncio.Task] = {}
+_cancel_reasons: dict[int, str] = {}
+
+
+def cancel_scan_task(scan_id: int) -> bool:
+    """Cancel the background task for a scan. Returns True if a task was found and cancelled."""
+    task = _running_tasks.get(scan_id)
+    if task and not task.done():
+        task.cancel()
+        return True
+    return False
+
+
+def pause_scan_task(scan_id: int) -> bool:
+    """Signal the running task to stop with status='paused'. Returns True if task was found."""
+    _cancel_reasons[scan_id] = 'paused'
+    return cancel_scan_task(scan_id)
+
+
+def stop_scan_task(scan_id: int) -> bool:
+    """Signal the running task to stop with status='cancelled'. Returns True if task was found."""
+    _cancel_reasons[scan_id] = 'cancelled'
+    return cancel_scan_task(scan_id)
+
 
 def _write_route_incremental(scan_id: int, destination: str,
                               dest_name: str, dest_city: str,
@@ -156,10 +184,10 @@ async def process_scan(scan_id: int):
 
         logger.info(f"[Scan {scan_id}] Scan details: {origin} -> {country_or_airports}, {start_date_str} to {end_date_str}")
 
-        # Update status to 'running'
+        # Update status to 'running' and record when processing started
         cursor.execute("""
             UPDATE scans
-            SET status = 'running', updated_at = CURRENT_TIMESTAMP
+            SET status = 'running', started_at = CURRENT_TIMESTAMP, updated_at = CURRENT_TIMESTAMP
             WHERE id = ?
         """, (scan_id,))
         conn.commit()
@@ -192,6 +220,7 @@ async def process_scan(scan_id: int):
                 UPDATE scans
                 SET status = 'failed',
                     error_message = ?,
+                    completed_at = CURRENT_TIMESTAMP,
                     updated_at = CURRENT_TIMESTAMP
                 WHERE id = ?
             """, (f"Failed to resolve airports: {str(e)}", scan_id))
@@ -294,11 +323,12 @@ async def process_scan(scan_id: int):
             "SELECT COALESCE(SUM(flight_count), 0) FROM routes WHERE scan_id = ?", (scan_id,)
         ).fetchone()[0]
 
-        # Update scan to completed
+        # Update scan to completed and record finish time
         cursor.execute("""
             UPDATE scans
             SET status = 'completed',
                 total_flights = ?,
+                completed_at = CURRENT_TIMESTAMP,
                 updated_at = CURRENT_TIMESTAMP
             WHERE id = ?
         """, (total_flights_saved, scan_id))
@@ -306,6 +336,24 @@ async def process_scan(scan_id: int):
 
         logger.info(f"[Scan {scan_id}] ✅ Scan completed successfully! {routes_saved} routes saved with {total_flights_saved} flights")
 
+    except asyncio.CancelledError:
+        reason = _cancel_reasons.pop(scan_id, 'cancelled')
+        logger.info(f"[Scan {scan_id}] Scan {reason} by user request")
+        try:
+            if conn:
+                cursor = conn.cursor()
+                cursor.execute("""
+                    UPDATE scans
+                    SET status = ?,
+                        completed_at = CURRENT_TIMESTAMP,
+                        updated_at = CURRENT_TIMESTAMP
+                    WHERE id = ?
+                """, (reason, scan_id))
+                conn.commit()
+        except Exception as update_error:
+            logger.error(f"[Scan {scan_id}] Failed to update {reason} status: {str(update_error)}")
+        raise  # must re-raise so asyncio marks the task as cancelled
+
     except Exception as e:
         logger.error(f"[Scan {scan_id}] ❌ Scan failed with error: {str(e)}", exc_info=True)
 
@@ -317,6 +365,7 @@ async def process_scan(scan_id: int):
                     UPDATE scans
                     SET status = 'failed',
                         error_message = ?,
+                        completed_at = CURRENT_TIMESTAMP,
                         updated_at = CURRENT_TIMESTAMP
                     WHERE id = ?
                 """, (str(e), scan_id))
@@ -340,5 +389,28 @@ def start_scan_processor(scan_id: int):
         asyncio.Task: The background task
     """
     task = asyncio.create_task(process_scan(scan_id))
+    _running_tasks[scan_id] = task
+    task.add_done_callback(lambda _: _running_tasks.pop(scan_id, None))
     logger.info(f"[Scan {scan_id}] Background task created")
     return task
+
+
+def start_resume_processor(scan_id: int):
+    """
+    Resume processing a paused scan as a background task.
+
+    The API endpoint has already reset status to 'pending' and cleared counters.
+    process_scan() will transition the status to 'running' and re-run all routes,
+    getting instant cache hits for already-queried routes.
+
+    Args:
+        scan_id: The ID of the paused scan to resume
+
+    Returns:
+        asyncio.Task: The background task
+    """
+    task = asyncio.create_task(process_scan(scan_id))
+    _running_tasks[scan_id] = task
+    task.add_done_callback(lambda _: _running_tasks.pop(scan_id, None))
+    logger.info(f"[Scan {scan_id}] Resume task created")
+    return task