fix(notes): page PhotoPrism server-side so /notes shows every captioned photo

Client-side paging of listPhotosWithNotes stopped early for BasePath users:
the sidecar post-filters each page by BasePath, so a full upstream page can
arrive short, tripping the `length < PAGE` end condition before the library
is exhausted — hiding notes past the first slice.

Add GET /api/sidecar/notes: the sidecar pages /api/v1/photos to completion
(keying the loop off the raw upstream page length), filters to non-empty
Caption under the caller's BasePath, dedupes by UID, and returns the set.
listPhotosWithNotes now calls this single endpoint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sidecar/main.go

@@ -108,6 +108,10 @@ func main() {
 									// tabs only show photos the user owns.
 									auth.GET("/timeline", handlePhotos(pp))
 
+									// Photos carrying a Note (Caption) — pages PhotoPrism fully so
+									// the /notes view isn't capped to the newest slice.
+									auth.GET("/notes", handleNotes(pp))
+
 									// User-scoped folders — post-filters the folder tree by BasePath
 									// so the sidebar shows only folders under the user's library root.
 									auth.GET("/folders", handleFoldersProxy(pp))