fix: guard migrations against fresh-database installs

On a fresh DB, migrations ran before the schema was applied and tried to
operate on tables that didn't exist yet (routes, scans), causing:
  "no such table: routes" on _migrate_add_routes_unique_index
  "no such table: scans" on _migrate_add_scheduled_scan_id_to_scans

Added table-existence checks so both migrations bail out when the table
isn't there yet. The schema's CREATE TABLE IF NOT EXISTS handles fresh
installs correctly.

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

flight-comparator/database/init_db.py

@@ -138,6 +138,13 @@ def _migrate_add_routes_unique_index(conn, verbose=True):
     Collapses any pre-existing duplicate (scan_id, destination) rows first
     (keeps the row with the lowest id) before creating the index.
     """
+    # Fresh install: routes table doesn't exist yet — schema will create the index
+    cursor = conn.execute(
+        "SELECT name FROM sqlite_master WHERE type='table' AND name='routes'"
+    )
+    if not cursor.fetchone():
+        return
+
     cursor = conn.execute(
         "SELECT name FROM sqlite_master WHERE type='index' AND name='uq_routes_scan_dest'"
     )
@@ -177,6 +184,8 @@ def _migrate_add_scheduled_scan_id_to_scans(conn, verbose=True):
     """
     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 column
     if 'scheduled_scan_id' in columns:
         return  # Already migrated