fix: check column existence before creating indexes

Fixes startup error 'no such column: session_id' that occurs when
_create_indexes() tries to create indexes on columns that don't exist yet.

The issue occurred when initializing a database from scratch:
1. _migrate_schema() adds session_id column to positions table
2. _create_indexes() tries to create index on session_id
3. But on fresh databases, positions table was created without session_id
4. Migration runs after table creation, before index creation
5. Index creation fails because column doesn't exist yet

Solution: Check if columns exist before creating indexes on them.

This ensures the database can be initialized both:
- Fresh (CREATE TABLE without session_id, then ALTER TABLE, then CREATE INDEX)
- Migrated (ALTER TABLE adds column, then CREATE INDEX)

Tested: All 21 database tests passing

api/database.py

@@ -438,12 +438,19 @@ def _create_indexes(cursor: sqlite3.Cursor) -> None:
     """)
 
     # Positions table - add index for simulation_run_id and session_id
-    cursor.execute("""
+    # Check if columns exist before creating indexes
-        CREATE INDEX IF NOT EXISTS idx_positions_run_id ON positions(simulation_run_id)
+    cursor.execute("PRAGMA table_info(positions)")
-    """)
+    position_columns = [row[1] for row in cursor.fetchall()]
-    cursor.execute("""
+
-        CREATE INDEX IF NOT EXISTS idx_positions_session_id ON positions(session_id)
+    if 'simulation_run_id' in position_columns:
-    """)
+        cursor.execute("""
+            CREATE INDEX IF NOT EXISTS idx_positions_run_id ON positions(simulation_run_id)
+        """)
+
+    if 'session_id' in position_columns:
+        cursor.execute("""
+            CREATE INDEX IF NOT EXISTS idx_positions_session_id ON positions(session_id)
+        """)
 
 
 def drop_all_tables(db_path: str = "data/jobs.db") -> None: