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/dd9f6a55604c_add_logo_column.py

@@ -0,0 +1,33 @@
+"""add_logo_column
+
+Add logo column to accounts table.
+
+Revision ID: dd9f6a55604c
+Revises: f854fd498a6e
+Create Date: 2025-09-30 23:16:35.530858
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "dd9f6a55604c"
+down_revision: Union[str, Sequence[str], None] = "f854fd498a6e"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Add logo column to accounts table."""
+    with op.batch_alter_table("accounts", schema=None) as batch_op:
+        batch_op.add_column(sa.Column("logo", sa.String(), nullable=True))
+
+
+def downgrade() -> None:
+    """Remove logo column."""
+    with op.batch_alter_table("accounts", schema=None) as batch_op:
+        batch_op.drop_column("logo")