feat(sync): server-side position authority — verify/heal progress anchors

Progress submissions now carry (percentage, context_text, epubcfi) and
the server becomes the position authority:

- VerifyProgressAnchor resolves the submitted standard CFI against the
  book's own XHTML, extracts the text at the anchor, and cross-checks it
  with the submitted context_text. A mismatch heals the anchor by text
  search (percentage disambiguates repeats) instead of storing a bad
  position.
- The anchor's block element is derived as a cssSelector plus a block-
  relative character offset, and served on progress GET alongside the
  anchor document's href — readium-native handles that let clients
  re-open a book without parsing CFIs themselves.
- context_text-only submissions (no CFI — the dumb-client tier) are
  anchored structurally from the context text.

Motivation: cross-client progress sync (web foliate CFIs, KOReader CRE
xpointers, readium-native apps) previously trusted each client's own
locator math; the app's EPUB restore drifted ±pages because readium's
paginator does not lay out far-from-viewport columns and the foliate-
ported CFI walk ran against readium's mutated WebView DOM. Server-side
verification heals both classes at ingest.

internal/sync/progress.go

@@ -274,8 +274,18 @@ func EstimatedPages(totalCharacters int64) int {
 type ProgressService struct {
 	db          *database.Queries
 	connManager *ConnectionManager
+	mediaPaths  MediaPathResolver
 }
 
+// MediaPathResolver resolves a media item's library-relative file path to
+// an absolute path, so the progress service can verify submitted anchors
+// against the book itself.
+type MediaPathResolver interface {
+	ResolveMediaPath(ctx context.Context, libraryID pgtype.UUID, relativePath string) (string, error)
+}
+
+func (s *ProgressService) SetMediaPathResolver(r MediaPathResolver) { s.mediaPaths = r }
+
 func NewProgressService(db *database.Queries, connManager *ConnectionManager) *ProgressService {
 	return &ProgressService{db: db, connManager: connManager}
 }
@@ -435,6 +445,39 @@ func (s *ProgressService) SaveProgress(ctx context.Context, req SaveProgressRequ
 		}
 	}
 
+	// Canonical position authority: resolve the submitted CFI against the
+	// book itself, cross-check the submitted context text, and heal the
+	// anchor by text search on any mismatch. Clients submit what their
+	// renderer can reliably observe (percentage + visible text); the
+	// server owns the structural math and keeps buggy clients from
+	// poisoning stored positions.
+	if !isFixed && s.mediaPaths != nil && params.ContextText.Valid &&
+		isConvertible(string(formatGroup)) && formatGroup != "" {
+		if path, perr := s.mediaPaths.ResolveMediaPath(ctx, mediaItem.LibraryID, mediaItem.FilePath); perr == nil && path != "" {
+			submittedCFI := ""
+			if params.Epubcfi.Valid {
+				submittedCFI = params.Epubcfi.String
+			}
+			submittedPct := 0.0
+			if params.Percentage.Valid {
+				submittedPct = params.Percentage.Float64
+			}
+			if finalCFI, _, _, charOff, healedPct, healed, verr := VerifyProgressAnchor(path, submittedCFI, params.ContextText.String, submittedPct); verr != nil {
+				log.Printf("Bookhoard: progress anchor verify failed for %s: %v", req.MediaItemID.String(), verr)
+			} else {
+				if healed || (!params.Epubcfi.Valid && finalCFI != "") {
+					params.Epubcfi = pgtype.Text{String: finalCFI, Valid: true}
+					if healedPct != nil {
+						params.Percentage = pgtype.Float8{Float64: *healedPct, Valid: true}
+					}
+					if charOff != nil {
+						params.CharacterOffset = pgtype.Int8{Int64: int64(*charOff), Valid: true}
+					}
+				}
+			}
+		}
+	}
+
 	conflictDetected := false
 	if hasExisting && existing.LastSyncSource.Valid && existing.LastSyncSource.String != req.Source {
 		if existing.LastSyncTimestamp.Valid {