feat: write routes live during scan instead of bulk-insert at completion

Routes and individual flights are now written to the database as each
query result arrives, rather than after all queries finish. The frontend
already polls /scans/:id/routes while status=running, so routes appear
progressively with no frontend changes needed.

Changes:
- database/schema.sql: UNIQUE INDEX uq_routes_scan_dest(scan_id, destination)
- database/init_db.py: _migrate_add_routes_unique_index() migration
- scan_processor.py: _write_route_incremental() helper; progress_callback
  now writes routes/flights immediately; Phase 2 bulk-write replaced with
  a lightweight totals query
- searcher_v3.py: pass flights= kwarg to progress_callback on cache_hit
  and api_success paths

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

flight-comparator/database/schema.sql

@@ -111,6 +111,10 @@ CREATE INDEX IF NOT EXISTS idx_routes_min_price
     ON routes(min_price)
     WHERE min_price IS NOT NULL;  -- Partial index for routes with prices
 
+-- One route row per (scan, destination) — enables incremental upsert writes
+CREATE UNIQUE INDEX IF NOT EXISTS uq_routes_scan_dest
+    ON routes(scan_id, destination);
+
 -- ============================================================================
 -- Triggers: Auto-update timestamps and aggregates
 -- ============================================================================