refactor(annotations): AnnotationService is a required constructor dependency

H1's doctrine applied to the annotation service, which had been left on
setters: NewMediaHandler/NewKoboHandler/NewKOReaderHandler/
NewSyncQueueProcessor(WithConfig) now take it as a parameter; the four
SetAnnotationService setters are deleted; wiring updated (main.go,
router/sync.go, test harness).

With the service unskippable, the 18 nil-guards and their plain-query
fallbacks die:
- media.go: the create fallbacks for notes/highlights/bookmarks (the
  bookmark one was the H3 census's Bug #1 — it dropped
  percentage/chapter_reference the sync path carries), the highlight
  PUT fallback, and the three delete-path plain deletes (tombstone is
  now the only delete semantics).
- kobo.go: both markup-ingest annotation loops (the bookmark fallback
  wrote the text into media_notes — a category error the fallback
  never should have had).
- koreader.go: all three push-stack guards.
- queue.go: the three "annotation service not available" errors in
  syncNote/syncHighlight/syncBookmark.

Behavior note: the deleted highlight create/PUT fallbacks were the only
paths passing note_id to the plain query — the sync service has no
NoteID field, so the web's highlight→note link was already being
dropped on the live path; this commit makes that pre-existing
limitation explicit rather than papering it over (surfacing note_id on
SaveHighlightRequest is left for a behavior commit that needs it).

Pins (now 20 incl. the 3 annotation CRUD suites) + H1 tests + both
unit suites green.

internal/sync/annotations.go

@@ -73,15 +73,15 @@ type SaveHighlightRequest struct {
 	// stored CFI drifts range→point shape after device echoes, and the
 	// user can edit the selection text — so a key-based upsert would mint
 	// a duplicate beside the very row being edited.
-	HighlightID       pgtype.UUID
-	PercentageStart   float64
-	PercentageEnd     float64
-	EpubcfiStart      string
-	EpubcfiEnd        string
-	ChapterReference  int32
-	Source            string
-	ModifiedAt        time.Time
-	DeviceSyncData    json.RawMessage
+	HighlightID      pgtype.UUID
+	PercentageStart  float64
+	PercentageEnd    float64
+	EpubcfiStart     string
+	EpubcfiEnd       string
+	ChapterReference int32
+	Source           string
+	ModifiedAt       time.Time
+	DeviceSyncData   json.RawMessage
 	// DedupKey overrides the computed key when the client echoes back an
 	// annotation it received from us (device echoes carry device-native
 	// locators, so the computed key would never match the original row and