Initial KOReader plugin: progress sync, annotations, device registration

Full feature set:
- In-plugin device registration with approval polling
- Reading progress sync (push on page update/close/suspend, pull on open/resume)
- Bookmark, highlight, and note sync via batch annotation collection
- Configurable sync behavior (silent/prompt/disable for forward/backward)
- OPDS catalog setup helper
- SHA-256 book hashing with doc_settings cache
- Debounced API calls with periodic push scheduling

_meta.lua

@@ -0,0 +1,5 @@
+local _ = require("gettext")
+return {
+    fullname = _("Bookhoard sync"),
+    description = _([[Synchronizes reading progress, bookmarks, highlights, and notes with your Bookhoard server.]]),
+}