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/4819c868ebc1_add_transaction_enrichments_table.py

@@ -0,0 +1,56 @@
+"""add_transaction_enrichments_table
+
+Add transaction_enrichments table for storing enriched transaction data.
+
+Revision ID: 4819c868ebc1
+Revises: dd9f6a55604c
+Create Date: 2025-09-30 23:20:00.969614
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "4819c868ebc1"
+down_revision: Union[str, Sequence[str], None] = "dd9f6a55604c"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Create transaction_enrichments table."""
+    op.create_table(
+        "transaction_enrichments",
+        sa.Column("accountId", sa.String(), nullable=False),
+        sa.Column("transactionId", sa.String(), nullable=False),
+        sa.Column("clean_name", sa.String(), nullable=True),
+        sa.Column("category", sa.String(), nullable=True),
+        sa.Column("logo_url", sa.String(), nullable=True),
+        sa.Column("created_at", sa.DateTime(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(), nullable=False),
+        sa.ForeignKeyConstraint(
+            ["accountId", "transactionId"],
+            ["transactions.accountId", "transactions.transactionId"],
+            ondelete="CASCADE",
+        ),
+        sa.PrimaryKeyConstraint("accountId", "transactionId"),
+    )
+
+    # Create indexes
+    op.create_index(
+        "idx_transaction_enrichments_category", "transaction_enrichments", ["category"]
+    )
+    op.create_index(
+        "idx_transaction_enrichments_clean_name",
+        "transaction_enrichments",
+        ["clean_name"],
+    )
+
+
+def downgrade() -> None:
+    """Drop transaction_enrichments table."""
+    op.drop_table("transaction_enrichments")