Add unified backend configuration system (#58)

Extract all hardcoded values (search terms, content filters, subgenre
patterns, quality weights, source endpoints) from fetch-data.js into
JSON config files under scripts/backend/config/. Add a typed config
loader (config.ts) with schema validation, sensible defaults, singleton
caching, and environment variable support.

Closes #48

Co-authored-by: Tasks Agent <tasks@localhost>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>