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/1ba02efe481c_migrate_to_composite_key.py

@@ -0,0 +1,102 @@
+"""migrate_to_composite_key
+
+Migrate transactions table to use composite primary key (accountId, transactionId).
+
+Revision ID: 1ba02efe481c
+Revises: bf30246cb723
+Create Date: 2025-09-30 23:16:34.637762
+
+"""
+
+from typing import Sequence, Union
+
+from sqlalchemy import text
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "1ba02efe481c"
+down_revision: Union[str, Sequence[str], None] = "bf30246cb723"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Migrate to composite primary key."""
+    conn = op.get_bind()
+
+    # Check if migration is needed
+    result = conn.execute(
+        text("""
+        SELECT name FROM sqlite_master
+        WHERE type='table' AND name='transactions'
+    """)
+    )
+
+    if not result.fetchone():
+        return
+
+    # Create temporary table with new schema
+    op.execute("""
+        CREATE TABLE transactions_temp (
+            accountId TEXT NOT NULL,
+            transactionId TEXT NOT NULL,
+            internalTransactionId TEXT,
+            institutionId TEXT NOT NULL,
+            iban TEXT,
+            transactionDate DATETIME,
+            description TEXT,
+            transactionValue REAL,
+            transactionCurrency TEXT,
+            transactionStatus TEXT,
+            rawTransaction JSON NOT NULL,
+            PRIMARY KEY (accountId, transactionId)
+        )
+    """)
+
+    # Insert deduplicated data (keep most recent duplicate)
+    op.execute("""
+        INSERT INTO transactions_temp
+        SELECT
+            accountId,
+            json_extract(rawTransaction, '$.transactionId') as transactionId,
+            internalTransactionId,
+            institutionId,
+            iban,
+            transactionDate,
+            description,
+            transactionValue,
+            transactionCurrency,
+            transactionStatus,
+            rawTransaction
+        FROM (
+            SELECT *,
+                   ROW_NUMBER() OVER (
+                       PARTITION BY accountId, json_extract(rawTransaction, '$.transactionId')
+                       ORDER BY transactionDate DESC, rowid DESC
+                   ) as rn
+            FROM transactions
+            WHERE json_extract(rawTransaction, '$.transactionId') IS NOT NULL
+            AND accountId IS NOT NULL
+        ) WHERE rn = 1
+    """)
+
+    # Replace tables
+    op.execute("DROP TABLE transactions")
+    op.execute("ALTER TABLE transactions_temp RENAME TO transactions")
+
+    # Recreate indexes
+    op.create_index(
+        "idx_transactions_internal_id", "transactions", ["internalTransactionId"]
+    )
+    op.create_index("idx_transactions_date", "transactions", ["transactionDate"])
+    op.create_index(
+        "idx_transactions_account_date",
+        "transactions",
+        ["accountId", "transactionDate"],
+    )
+    op.create_index("idx_transactions_amount", "transactions", ["transactionValue"])
+
+
+def downgrade() -> None:
+    """Not implemented - would require changing primary key back."""