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/init_db.py

@@ -130,6 +130,43 @@ def _migrate_relax_country_constraint(conn, verbose=True):
         print("   ✅ Migration complete: country column now accepts >= 2 chars")
 
 
+def _migrate_add_routes_unique_index(conn, verbose=True):
+    """
+    Migration: Add UNIQUE index on routes(scan_id, destination).
+
+    Required for incremental route writes during active scans.
+    Collapses any pre-existing duplicate (scan_id, destination) rows first
+    (keeps the row with the lowest id) before creating the index.
+    """
+    cursor = conn.execute(
+        "SELECT name FROM sqlite_master WHERE type='index' AND name='uq_routes_scan_dest'"
+    )
+    if cursor.fetchone():
+        return  # Already migrated
+
+    if verbose:
+        print("   🔄 Migrating routes table: adding UNIQUE index on (scan_id, destination)...")
+
+    # Collapse any existing duplicates (guard against edge cases)
+    conn.execute("""
+        DELETE FROM routes
+        WHERE id NOT IN (
+            SELECT MIN(id)
+            FROM routes
+            GROUP BY scan_id, destination
+        )
+    """)
+
+    conn.execute("""
+        CREATE UNIQUE INDEX IF NOT EXISTS uq_routes_scan_dest
+        ON routes(scan_id, destination)
+    """)
+    conn.commit()
+
+    if verbose:
+        print("   ✅ Migration complete: uq_routes_scan_dest index created")
+
+
 def initialize_database(db_path=None, verbose=True):
     """
     Initialize or migrate the database.
@@ -174,6 +211,7 @@ def initialize_database(db_path=None, verbose=True):
 
     # Apply migrations before running schema
     _migrate_relax_country_constraint(conn, verbose)
+    _migrate_add_routes_unique_index(conn, verbose)
 
     # Load and execute schema
     schema_sql = load_schema()