Fix DB integration quality: dependency injection, idempotent seeding, price precision, and test isolation

[Copilot]

Addresses all review comments on the PostgreSQL integration PR: per-request DB connections leaking, duplicate router declaration, ON CONFLICT DO NOTHING being a no-op (no unique constraint), price losing precision via float(), scripts executing at import time, missing dependencies in requirements.txt, and tests requiring a live DB.

API / Routes (app/api/routes.py)

• Remove duplicate router = APIRouter()
• Move load_dotenv() to module level
• Extract DB setup into a get_db_connection() generator dependency injected via Depends() — connection lifecycle guaranteed by try/finally
• Price serialised as str(row[3]) instead of float(row[3]) to preserve NUMERIC exactness

def get_db_connection():
    conn = psycopg2.connect(...)
    try:
        yield conn
    finally:
        conn.close()

@router.get("/")
def root(conn=Depends(get_db_connection)) -> dict:
    ...

Seeding (app/seed_product_table.py)

• Add UNIQUE constraint on name so ON CONFLICT (name) DO NOTHING is a real conflict target — re-running the script is now idempotent
• Wrap execution in main() + if __name__ == "__main__": guard

Utility scripts (app/print_products.py, app/test_db_connection.py)

• Wrap in main() + guard to prevent side effects on import
• test_db_connection.py returns a non-zero exit code on failure via sys.exit()

Dependencies (requirements.txt)

• Add python-dotenv>=1.0.0 and psycopg2-binary>=2.9.0

Metadata (app/main.py)

• Source FastAPI version from app.__version__ instead of the hard-coded "1.0.0"

Tests (tests/test_routes.py)

• Override get_db_connection with a mock dependency — tests run without a live PostgreSQL instance
• Replace stale test_root_returns_welcome_message with tests validating the {meta, data} response shape; mock rows use Decimal to accurately reflect DB types

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[goldlabelapps]

LGTM