docs(api): real progress endpoint contract; remove nonexistent-endpoint docs

The progress documentation described GET/POST /api/progress/:id —
endpoints that do not exist in the router — while the endpoint every
client actually uses (GET/PUT/DELETE /api/media-items/:id/progress) had
no field-level docs at all.

New:
- progress/position-contract.md: the canonical position model — server
  as position authority, the three-tier submission (percentage /
  context_text / epubcfi), ingest verification and healing, the restore
  handles, the OPF spine numbering hazard (canonical CFI spine steps
  include linear="no" items; clients resolve documents by anchor_href
  and land by css_selector + char_offset, never by spine step), the
  offset currencies (UTF-16 at the wire, runes internal), and
  context_text rules.
- progress/get_media_progress.md and update_media_progress.md: the real
  endpoints with full field tables, conditionality of the restore
  handles, the first-page anti-clobber guard, and healed-response
  semantics.
- progress/delete_media_progress.md: the real DELETE route.
- koreader/sync_progress.md and koreader/get_metadata.md rewritten to
  the actual payloads: the plugin sends a single-book array whose
  "epubcfi" field is a CRE xpointer; the metadata response navigates
  via koreader_xpointer (canonical CFI converted back to CRE), with
  page as the canonical locator for fixed-layout books.

Removed: the five files documenting the nonexistent /api/progress/:id
GET/POST/DELETE endpoints. Kept get_progress_history.md (that route
exists). The legacy developer/api-reference.md and the indexed
api/api-reference.md progress sections now match the wire and link the
new docs; the duplicate "Universal Progress" section points at Reading
Progress.

docs/developer/api/progress/delete_media_progress.md

@@ -0,0 +1,35 @@
+# Delete Media Item Progress
+
+Delete the stored reading progress for a media item.
+
+**Endpoint**: `DELETE /api/media-items/:id/progress`
+**Auth**: Required (Bearer token)
+
+## Path Parameters
+
+| Parameter | Type          | Required | Description     |
+| --------- | ------------- | -------- | ---------------- |
+| id        | string (UUID) | Yes      | Media item UUID |
+
+### Example Request
+
+```http
+DELETE /api/media-items/774641f9-317b-4087-8e04-53bb4392ae56/progress
+Authorization: Bearer eyJhbGciOiJIUzI1NiIs...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "reading progress deleted"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+| ---- | ----------- |
+| 400  | Invalid media item id |
+| 401  | Invalid or expired token |
+| 500  | Database error |