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/f854fd498a6e_add_sync_operations_table.py

@@ -0,0 +1,59 @@
+"""add_sync_operations_table
+
+Add sync_operations table for tracking synchronization operations.
+
+Revision ID: f854fd498a6e
+Revises: be8d5807feca
+Create Date: 2025-09-30 23:16:35.229062
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "f854fd498a6e"
+down_revision: Union[str, Sequence[str], None] = "be8d5807feca"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Create sync_operations table."""
+    op.create_table(
+        "sync_operations",
+        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column("started_at", sa.DateTime(), nullable=False),
+        sa.Column("completed_at", sa.DateTime(), nullable=True),
+        sa.Column("success", sa.Boolean(), nullable=True),
+        sa.Column(
+            "accounts_processed", sa.Integer(), nullable=False, server_default="0"
+        ),
+        sa.Column(
+            "transactions_added", sa.Integer(), nullable=False, server_default="0"
+        ),
+        sa.Column(
+            "transactions_updated", sa.Integer(), nullable=False, server_default="0"
+        ),
+        sa.Column("balances_updated", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("duration_seconds", sa.Float(), nullable=True),
+        sa.Column("errors", sa.String(), nullable=True),
+        sa.Column("logs", sa.String(), nullable=True),
+        sa.Column("trigger_type", sa.String(), nullable=False, server_default="manual"),
+        sa.PrimaryKeyConstraint("id"),
+    )
+
+    # Create indexes
+    op.create_index("idx_sync_operations_started_at", "sync_operations", ["started_at"])
+    op.create_index("idx_sync_operations_success", "sync_operations", ["success"])
+    op.create_index(
+        "idx_sync_operations_trigger_type", "sync_operations", ["trigger_type"]
+    )
+
+
+def downgrade() -> None:
+    """Drop sync_operations table."""
+    op.drop_table("sync_operations")