Replace manual If-None-Match comparison with Response::setEtag() and
Response::isNotModified(), which properly handles ETag quoting, weak
vs strong comparison, and 304 response cleanup. Fixes PHPStan return
type error and CI test failures.
- Use empty Response instead of JsonResponse(null) for 304 Not Modified
to avoid sending "null" as response body
- Guard parse_url() result with is_string() since it can return false
for malformed URLs
- Move storage location tracking inside the availability check so
expired and unknown-quantity lots don't contribute locations
- Add Stock field showing total available quantity across all part lots
- Add Storage Location field when parts have stored locations
- Resolve actual datasheet PDF from attachments (by type name, attachment
name, or first PDF) instead of always linking to Part-DB page
- Keep Part-DB page URL as separate "Part-DB URL" field
- Add ETag and Cache-Control headers to all KiCad API endpoints
- Support conditional requests (If-None-Match) returning 304
- Categories/part lists cached 5 min, part details cached 1 min
