feat(bookmarks): location identity, origin provenance, KOReader-style labels

- Drop UNIQUE(media_item_id,user_id,title): titles are display labels
  shared verbatim across clients; same-title bookmarks on different pages
  now coexist instead of 500ing (deleting over a tombstone no longer
  blocks future creates with that title)
- Add origin_source column recording the creating client (android/web/
  koreader), set once at insert, exposed in API responses
- Web reader auto-title mirrors KOReader's 'in <chapter>' convention,
  falling back to 'Bookmark'; adds bookmark rename in the drawer

templates/reader.templ

@@ -730,10 +730,33 @@ templ ReaderAnnotationsDrawer() {
 						href="#"
 						@click.prevent="goToBookmark(bookmark)"
 						class="flex-1 min-w-0 block py-2 hover:bg-gray-700 rounded px-2"
+						x-show="!bookmark.renameOpen"
 					>
 						<span class="font-medium block truncate" x-text="bookmark.title"></span>
 						<span class="text-xs block truncate" style="color: var(--text-secondary)" x-text="bookmark.positionLabel"></span>
 					</a>
+					<div class="flex-1 min-w-0 py-2" x-show="bookmark.renameOpen" @click.stop>
+						<input
+							type="text"
+							class="reader-note-input"
+							x-model="bookmark.renameText"
+							@keydown.enter="renameBookmark(bookmark)"
+							@keydown.escape="bookmark.renameOpen = false"
+							placeholder="Bookmark name…"
+						></input>
+						<div class="flex gap-2 mt-1">
+							<button @click="renameBookmark(bookmark)" class="flex-1 py-1 text-xs bg-blue-600 text-white rounded hover:bg-blue-700">Save</button>
+							<button @click="bookmark.renameOpen = false" class="flex-1 py-1 text-xs border rounded hover:opacity-80" style="border-color: var(--border);">Cancel</button>
+						</div>
+					</div>
+					<button
+						@click="startBookmarkRename(bookmark)"
+						class="p-2 rounded hover:bg-gray-600 opacity-0 group-hover:opacity-100 transition-opacity"
+						title="Rename bookmark"
+						aria-label="Rename bookmark"
+					>
+						✎
+					</button>
 					<button
 						@click="deleteBookmark(bookmark.id)"
 						class="p-2 rounded hover:bg-red-900/60 opacity-0 group-hover:opacity-100 transition-opacity"