Refactor configuration to use one source of truth for base URL
- Add GetBaseURL() to config package: queries system_config table first,
falls back to BASE_URL env var
- Update SidecarHandler to accept config and use single base_url
- Compute opds/api paths from base_url instead of storing separately:
- OPDS: base_url + /opds
- API: base_url + /api
- Device Sync: base_url + /api/sync
- Simplify OPDSHandler.getBaseURLs() to compute opds path
- Remove need for separate opds_base_url and api_base_url columns
Previously the system stored three separate URL config values that were
usually the same domain with different paths. Now store only base_url
and compute the paths, eliminating configuration redundancy.