feat(pdftract-4z362): implement inspector API endpoints
- Added api.rs module with handlers for /api/document, /api/page/{i}, /api/page/{i}/svg,
/api/page/{i}/thumbnail, /api/raster/{i}.png, and /api/search
- Implemented Bearer token authentication for non-loopback binds
- Added base64 dependency for raster PNG decoding
- Returns 404 for /api/raster on vector pages (no raster field)
- Search performs case-insensitive substring matching across all spans
- SVG rendering is placeholder pending full renderer integration
Closes: pdftract-4z362
1cf026ace7