91 fix database initialization and seeding

[krgjert]

Purpose

Allow database initialization and reseeding to be rerun safely without deleting existing published builds.

Changes

• Use IF NOT EXISTS when creating the builds table.
• Replace TRUNCATE with a temporary import table and upsert components by ID.
• Preserve existing builds and components missing from the CSV.
• Run initialization and seeding in one transaction, stopping on errors.
• Validate DATABASE_URL and document reseeding in README.

Related issues

Closes #91

Verification

Manually tested on local PostgreSQL 16.15:

• Initial setup imported 800 components.
• Running bash backend/db/seed.sh again succeeded.
• Reseeding preserved an existing build and its CPU reference.
• A modified price was restored from the CSV, with 800 components remaining.
• A deliberate SQL error rolled back the transaction, preserving the previous price.

No automated tests were added; verification used a separate local test database.

Screenshots

Not applicable.

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.

[wilhelsr]

LGTM. Nice improvements to database seeding and data preservation.