feat(series): post→series flows + browse list — backend (FC-6.2)

The post-aware on-ramp + the data behind the missing Series browse view.

- page_number_parser: conservative stated-page parser (pages 9-12 / page 5 /
  [3/8] / 3 of 8), keyword-gated to avoid false positives. Pure + unit-tested.
- SeriesService.promote_post_to_series: a self-contained post becomes its own
  series — series tag named after the post, one chapter, the post's images as
  pages (ordered by capture order; stated pages parsed from title/description).
- SeriesService.add_post_as_chapter: append a post as the next chapter of an
  existing series, titled after the post and slotted by parsed page number
  (a "pages 1-4" post lands ahead of the "pages 9-12" chapter).
- SeriesService.list_series: browse cards — cover thumb, artist, chapter/page
  counts, gap flag, last-updated; sort recent|name|size + filter by artist.
- API: GET /api/series, POST /api/series/from-post, POST /api/series/<id>/add-post.
- Resolver uses ImageRecord.primary_post_id (same linkage the posts feed renders).

Frontend (Add-to-series control + Series view + nav) lands next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/tags.py

@@ -597,3 +597,59 @@ async def series_chapter_reorder_pages(tag_id: int, chapter_id: int):
             return _series_err(exc)
         await session.commit()
     return jsonify({"ok": True})
+
+
+# ---- browse list + post→series flows (FC-6.2) -----------------------------
+
+
+@tags_bp.route("/series", methods=["GET"])
+async def series_list():
+    args = request.args
+    sort = args.get("sort", "recent")
+    if sort not in ("recent", "name", "size"):
+        return jsonify({"error": "sort must be recent|name|size"}), 400
+    artist_id = None
+    if args.get("artist_id") is not None:
+        try:
+            artist_id = int(args["artist_id"])
+        except ValueError:
+            return jsonify({"error": "artist_id must be an integer"}), 400
+    async with get_session() as session:
+        rows = await SeriesService(session).list_series(
+            sort=sort, artist_id=artist_id
+        )
+    return jsonify({"series": rows})
+
+
+@tags_bp.route("/series/from-post", methods=["POST"])
+async def series_from_post():
+    body = await request.get_json()
+    post_id, err = _opt_int(body, "post_id")
+    if err:
+        return err
+    if post_id is None:
+        return jsonify({"error": "post_id required"}), 400
+    async with get_session() as session:
+        try:
+            out = await SeriesService(session).promote_post_to_series(post_id)
+        except SeriesError as exc:
+            return _series_err(exc)
+        await session.commit()
+    return jsonify(out)
+
+
+@tags_bp.route("/series/<int:tag_id>/add-post", methods=["POST"])
+async def series_add_post(tag_id: int):
+    body = await request.get_json()
+    post_id, err = _opt_int(body, "post_id")
+    if err:
+        return err
+    if post_id is None:
+        return jsonify({"error": "post_id required"}), 400
+    async with get_session() as session:
+        try:
+            out = await SeriesService(session).add_post_as_chapter(tag_id, post_id)
+        except SeriesError as exc:
+            return _series_err(exc)
+        await session.commit()
+    return jsonify(out)