feat(server): progress locator coherence — percentage-only writes clear stale anchors

The verify/heal pipeline verified only INBOUND context-bearing writes;
stored-row coherence was never enforced. SaveProgress seeds missing
fields from the existing row, so every percentage-only write (KOReader
low-confidence conversion, KOReader no-pointer payloads, Kobo markup
ReadingSync / analytics / sync-from-server, locator-less queue entries)
silently re-paired its NEW percentage with the PREVIOUS writer's stale
CFI+context — the incoherent row shape that fed the client-side
far-anchor restore floor (and, via the read-side anchor handles, could
even hand a first-party client the OLD position's scroll address).

KOReader's converter already declared the right intent ("storing
percentage only — the row's percentage restores approximately"); the
seeding betrayed it. SaveProgressRequest gains ClearLocator, set by the
percentage-only call sites, and the seeding rule honors it: stale
epubcfi/context/character_offset clear (the upsert assigns columns
directly, so NULL sticks) unless the request carries its own. A
request-carried context survives so the verify/heal pass can still mint
a fresh anchor from it; the HTTP merge path (field-absent PUTs
preserve) and all anchor-bearing writes are untouched. The fixed-layout
branch already cleared locators unconditionally — unchanged.

Consumer audit: percentage — the only thing every sync round-trip
exchanges — is untouched on all paths; KOReader pulls on locator-less
rows already support the nil-CFI shape (builder guards
progress.Epubcfi.Valid); first-party restores go percentage-first
instead of floor-fighting a stale anchor.

Tests: TestProgressWeb_LocatorCoherence pins clear-on-declared,
request-carried-context survival, and merge-path preservation. Full
suite with the change: 218 groups, zero failures.

internal/handlers/kobo.go

@@ -254,9 +254,9 @@ type KoboInitResponse struct {
 }
 
 type KoboSyncStatus struct {
-	Status            string                 `json:"Status"`
-	MarkupsSynced     int                    `json:"MarkupsSynced"`
-	BookmarksSynced   int                    `json:"BookmarksSynced"`
+	Status             string                  `json:"Status"`
+	MarkupsSynced      int                     `json:"MarkupsSynced"`
+	BookmarksSynced    int                     `json:"BookmarksSynced"`
 	DeletedAnnotations []KoboDeletedAnnotation `json:"DeletedAnnotations,omitempty"`
 }
 
@@ -448,16 +448,17 @@ func (h *KoboHandler) Markup(c *echo.Context) error {
 
 		if h.progressSvc != nil {
 			_, err = h.progressSvc.SaveProgress(c.Request().Context(), wsync.SaveProgressRequest{
-				MediaItemID: pgMediaUUID,
-				UserID:      pgUserID,
-				Source:      "kobo",
-				DeviceID:    pgtype.UUID{Bytes: deviceID, Valid: true},
-				Percentage:  &percentage,
-				CurrentPage: currentPage,
-				TotalPages:  totalPages,
-				DeviceType:  "kobo",
-				DeviceName:  device.DeviceName,
-				Broadcast:   true,
+				MediaItemID:  pgMediaUUID,
+				UserID:       pgUserID,
+				Source:       "kobo",
+				DeviceID:     pgtype.UUID{Bytes: deviceID, Valid: true},
+				Percentage:   &percentage,
+				CurrentPage:  currentPage,
+				TotalPages:   totalPages,
+				DeviceType:   "kobo",
+				DeviceName:   device.DeviceName,
+				Broadcast:    true,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{
@@ -798,14 +799,15 @@ func (h *KoboHandler) AnalyticsGettests(c *echo.Context) error {
 
 		if h.progressSvc != nil {
 			_, err = h.progressSvc.SaveProgress(c.Request().Context(), wsync.SaveProgressRequest{
-				MediaItemID: pgMediaUUID,
-				UserID:      pgUserID,
-				Source:      "kobo",
-				DeviceID:    pgtype.UUID{Bytes: deviceID, Valid: true},
-				Percentage:  &percentage,
-				DeviceType:  "kobo",
-				DeviceName:  device.DeviceName,
-				Broadcast:   true,
+				MediaItemID:  pgMediaUUID,
+				UserID:       pgUserID,
+				Source:       "kobo",
+				DeviceID:     pgtype.UUID{Bytes: deviceID, Valid: true},
+				Percentage:   &percentage,
+				DeviceType:   "kobo",
+				DeviceName:   device.DeviceName,
+				Broadcast:    true,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{
@@ -860,14 +862,15 @@ func (h *KoboHandler) SyncFromServer(c *echo.Context) error {
 
 		if h.progressSvc != nil {
 			_, err = h.progressSvc.SaveProgress(c.Request().Context(), wsync.SaveProgressRequest{
-				MediaItemID: pgMediaUUID,
-				UserID:      pgUserID,
-				Source:      "bookhoard",
-				DeviceID:    pgtype.UUID{Bytes: deviceID, Valid: true},
-				Percentage:  &percentage,
-				DeviceType:  "kobo",
-				DeviceName:  device.DeviceName,
-				Broadcast:   false,
+				MediaItemID:  pgMediaUUID,
+				UserID:       pgUserID,
+				Source:       "bookhoard",
+				DeviceID:     pgtype.UUID{Bytes: deviceID, Valid: true},
+				Percentage:   &percentage,
+				DeviceType:   "kobo",
+				DeviceName:   device.DeviceName,
+				Broadcast:    false,
+				ClearLocator: true,
 			})
 		} else {
 			_, err = h.db.UpdateUniversalProgress(c.Request().Context(), database.UpdateUniversalProgressParams{