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/handlers/media.go

@@ -953,6 +953,37 @@ func (mh *MediaHandler) GetMediaReadingProgress(c *echo.Context) error {
 		"last_sync_timestamp": progress.LastSyncTimestamp,
 	}
 
+	// Position authority: re-verify the stored anchor and derive the
+	// client-facing handles (cssSelector + anchor document href) server-
+	// side, so clients scroll to an address instead of parsing CFIs.
+	if progress.Epubcfi.Valid && wsync.IsConvertibleFormat(progress.FormatGroup) && mh.libraryService != nil {
+		contextText := ""
+		if progress.ContextText.Valid {
+			contextText = progress.ContextText.String
+		}
+		pct := 0.0
+		if progress.Percentage.Valid {
+			pct = progress.Percentage.Float64
+		}
+		if mediaItem, err := mh.db.GetMediaItem(c.Request().Context(), pgtype.UUID{Bytes: mediaUUID, Valid: true}); err == nil {
+			if path, perr := mh.libraryService.ResolveMediaPath(c.Request().Context(), mediaItem.LibraryID, mediaItem.FilePath); perr == nil && path != "" {
+				if finalCFI, sel, href, charOff, _, _, verr := wsync.VerifyProgressAnchor(path, progress.Epubcfi.String, contextText, pct); verr == nil && sel != "" {
+					resp["css_selector"] = sel
+					resp["anchor_href"] = href
+					if charOff != nil {
+						resp["char_offset"] = *charOff
+					}
+					if finalCFI != "" {
+						resp["epubcfi"] = finalCFI
+					}
+				}
+			}
+		}
+	}
+	if progress.ContextText.Valid {
+		resp["context_text"] = progress.ContextText.String
+	}
+
 	return c.JSON(http.StatusOK, resp)
 }