[ chebizarro/nostrc ] refactor: move profile provider and service to nostr-gobject (nostrc-lx24)

Move gnostr-profile-provider and gnostr-profile-service from
apps/gnostr/ into nostr-gobject/src/ as library-level services.
These are pure GObject/GLib code with no GTK dependencies.

- Create nostr_utils.h/c with gnostr_ensure_hex_pubkey (was in app utils.c)
- Create nostr_profile_provider.h/c with LRU cache and NDB-backed provider
- Create nostr_profile_service.h/c with batched relay fetch service
- Replace app-specific deps with registerable callbacks:
https://github.com/chebizarro/nostrc/commit/3204a782b142e1a7d62162dcf20aa4403acf13cb