refactor: Migrate database service to SQLModel and Alembic.

- Add SQLModel for type-safe database models
- Implement Alembic for schema migration management
- Create 7 migrations covering all existing schema changes
- Add automatic migration system that runs on startup
- Maintain backward compatibility with existing raw SQL queries
- Remove old manual migration system
- All tests pass (109 tests)

Benefits:
- Full type safety with Pydantic validation
- Version-controlled schema changes
- Automatic migration detection and application
- Better developer experience with typed models

alembic/versions/bf30246cb723_migrate_balance_timestamps.py

@@ -0,0 +1,62 @@
+"""migrate_balance_timestamps
+
+Convert Unix timestamps to datetime strings in balances table.
+
+Revision ID: bf30246cb723
+Revises: de8bfb1169d4
+Create Date: 2025-09-30 23:14:03.128959
+
+"""
+
+from datetime import datetime
+from typing import Sequence, Union
+
+from sqlalchemy import text
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "bf30246cb723"
+down_revision: Union[str, Sequence[str], None] = "de8bfb1169d4"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Convert all Unix timestamps to datetime strings."""
+    conn = op.get_bind()
+
+    # Get all balances with REAL timestamps
+    result = conn.execute(
+        text("""
+        SELECT id, timestamp
+        FROM balances
+        WHERE typeof(timestamp) = 'real'
+        ORDER BY id
+    """)
+    )
+
+    unix_records = result.fetchall()
+
+    if not unix_records:
+        return
+
+    # Convert and update in batches
+    for record_id, unix_timestamp in unix_records:
+        try:
+            # Convert Unix timestamp to datetime string
+            dt_string = datetime.fromtimestamp(float(unix_timestamp)).isoformat()
+
+            # Update the record
+            conn.execute(
+                text("UPDATE balances SET timestamp = :dt WHERE id = :id"),
+                {"dt": dt_string, "id": record_id},
+            )
+        except Exception:
+            continue
+
+    conn.commit()
+
+
+def downgrade() -> None:
+    """Not implemented - converting back would lose precision."""