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/get_media_progress.md

@@ -0,0 +1,98 @@
+# Get Media Item Progress
+
+Get the stored reading progress for a media item, plus the
+server-derived restore handles for reflowable books.
+
+**Endpoint**: `GET /api/media-items/:id/progress`
+**Auth**: Required (Bearer token)
+
+See [Position Contract](position-contract.md) for the semantics of every
+field — what is verified, what the currencies are, and how clients
+should restore.
+
+## Path Parameters
+
+| Parameter | Type          | Required | Description     |
+| --------- | ------------- | -------- | --------------- |
+| id        | string (UUID) | Yes      | Media item UUID |
+
+### Example Request
+
+```http
+GET /api/media-items/774641f9-317b-4087-8e04-53bb4392ae56/progress
+Authorization: Bearer eyJhbGciOiJIUzI1NiIs...
+```
+
+## Response (200 OK) — no progress stored
+
+When the item has no progress row, an empty fixed-layout-style stub is
+returned (not 404):
+
+```json
+{
+  "current_page": 0,
+  "total_pages": null
+}
+```
+
+## Response (200 OK) — progress stored
+
+```json
+{
+  "id": "e8239659-0ed6-42ae-a946-8440d7655b42",
+  "media_item_id": "774641f9-317b-4087-8e04-53bb4392ae56",
+  "user_id": "1b64992e-3408-4d84-9e03-e2dc4950e1dd",
+  "current_page": null,
+  "total_pages": null,
+  "last_read_at": "2026-09-26T14:52:34.806446Z",
+  "percentage": 0.045,
+  "character_offset": 16375,
+  "epubcfi": "epubcfi(/6/4!/4/8[_idContainer003]/58/1:598)",
+  "chapter": null,
+  "chapter_progress": null,
+  "format_group": "reflowable",
+  "total_characters": 592216,
+  "chapter_count": 1,
+  "last_sync_device": "web",
+  "last_sync_source": "koreader",
+  "last_sync_timestamp": "2026-09-26T14:52:34.806446Z",
+  "css_selector": "body>div:nth-child(4)>p:nth-child(29)",
+  "anchor_href": "1984.xhtml",
+  "char_offset": 598,
+  "context_text": "from day to day, but there was none in which Goldstein was not the principal figure. He was the prim"
+}
+```
+
+### Field Reference
+
+Base fields (always present when a row exists):
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `percentage` | float | Position as a fraction of the whole book (0..1). |
+| `epubcfi` | string | The stored canonical standard CFI. **Spine steps index the OPF spine as written, including `linear="no"` items — do not resolve them against a readium reading order.** See the [Position Contract](position-contract.md#spine-numbering-hazard--read-this-before-parsing-a-stored-cfi). |
+| `context_text` | string | The stored verification context (≤100 whitespace-normalized chars from the anchor). |
+| `character_offset` | int | Book-wide rune offset. Internal currency — consistent with `total_characters`. |
+| `current_page`, `total_pages` | int | Fixed-layout page position (null for reflowable). |
+| `chapter`, `chapter_progress` | int, float | Chapter index and within-chapter fraction, when known. |
+| `format_group` | string | `reflowable`, `fixed_layout`, `comic_archive`, … Gates the restore handles. |
+| `total_characters`, `chapter_count` | int | Book metrics, for client-side fraction math. |
+| `last_sync_device`, `last_sync_source`, `last_sync_timestamp` | — | Which client last wrote the row. |
+
+Restore handles (conditional — served only for convertible reflowable
+books whose stored anchor re-resolves at GET time):
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `anchor_href` | string | The spine document containing the anchor (e.g. `1984.xhtml`). **Resolve your resource by this**, never by the CFI's spine step. |
+| `css_selector` | string | Body-relative chain to the anchor's block element. |
+| `char_offset` | int | Anchor offset within the block's concatenated text, in UTF-16 code units. |
+| `epubcfi` | string | Re-served (possibly healed) canonical CFI — present whenever re-verification produced one. |
+
+## Error Responses
+
+| Code | Description |
+| ---- | ----------- |
+| 400  | Invalid media item id |
+| 401  | Invalid or expired token |
+| 500  | Database error |