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>

tests/test_series_from_post.py

@@ -0,0 +1,115 @@
+"""FC-6.2 — promote a post to a series + add a post as a chapter + browse list."""
+
+import pytest
+
+from backend.app.models import Artist, ImageRecord, Post, TagKind
+from backend.app.services.series_service import SeriesService
+from backend.app.services.tag_service import TagService
+
+pytestmark = pytest.mark.integration
+
+_SEQ = 0
+
+
+async def _artist(db, name):
+    a = Artist(name=name, slug=name.lower().replace(" ", "-"))
+    db.add(a)
+    await db.flush()
+    return a
+
+
+async def _post(db, artist, title, ext):
+    p = Post(artist_id=artist.id, external_post_id=ext, post_title=title)
+    db.add(p)
+    await db.flush()
+    return p
+
+
+async def _post_images(db, post, artist, n):
+    global _SEQ
+    ids = []
+    for _ in range(n):
+        _SEQ += 1
+        rec = ImageRecord(
+            path=f"/tmp/fc_ps_{_SEQ}.png", sha256=f"{_SEQ:064d}",
+            size_bytes=1, mime="image/png", origin="downloaded",
+            primary_post_id=post.id, artist_id=artist.id,
+        )
+        db.add(rec)
+        await db.flush()
+        ids.append(rec.id)
+    return ids
+
+
+@pytest.mark.asyncio
+async def test_promote_post_to_series(db):
+    svc = SeriesService(db)
+    artist = await _artist(db, "Bikupan")
+    post = await _post(db, artist, "My Comic pages 1-3", "pp1")
+    imgs = await _post_images(db, post, artist, 3)
+
+    out = await svc.promote_post_to_series(post.id)
+    assert out["added"] == 3
+    assert out["name"] == "My Comic pages 1-3"
+
+    data = await svc.list_pages(out["series_tag_id"])
+    assert len(data["chapters"]) == 1
+    ch = data["chapters"][0]
+    assert ch["stated_page_start"] == 1
+    assert ch["stated_page_end"] == 3
+    assert [p["image_id"] for p in ch["pages"]] == imgs  # capture order
+    assert [p["stated_page"] for p in ch["pages"]] == [1, 2, 3]
+
+
+@pytest.mark.asyncio
+async def test_promote_requires_images(db):
+    svc = SeriesService(db)
+    artist = await _artist(db, "Empty Artist")
+    post = await _post(db, artist, "No images", "pp2")
+    from backend.app.services.series_service import SeriesError
+    with pytest.raises(SeriesError):
+        await svc.promote_post_to_series(post.id)
+
+
+@pytest.mark.asyncio
+async def test_add_post_as_chapter_slots_by_stated_page(db):
+    svc = SeriesService(db)
+    artist = await _artist(db, "Story Artist")
+    sid = (await TagService(db).find_or_create("Story", TagKind.series)).id
+    # An existing chapter stating pages 9-12.
+    await svc.create_chapter(sid, stated_page_start=9, stated_page_end=12)
+    # A post stating pages 1-4 should slot BEFORE the 9-12 chapter.
+    post = await _post(db, artist, "Story pages 1-4", "pp3")
+    await _post_images(db, post, artist, 4)
+    out = await svc.add_post_as_chapter(sid, post.id)
+    assert out["added"] == 4
+
+    data = await svc.list_pages(sid)
+    chapters = data["chapters"]
+    assert chapters[0]["stated_page_start"] == 1   # new one is first
+    assert chapters[1]["stated_page_start"] == 9
+    # gap 5-8 flagged between them
+    assert data["gaps"][0]["start"] == 5
+    assert data["gaps"][0]["end"] == 8
+
+
+@pytest.mark.asyncio
+async def test_list_series_cards(db):
+    svc = SeriesService(db)
+    artist = await _artist(db, "Card Artist")
+    post = await _post(db, artist, "Card Comic pages 1-2", "pp4")
+    await _post_images(db, post, artist, 2)
+    out = await svc.promote_post_to_series(post.id)
+
+    rows = await svc.list_series()
+    card = next(r for r in rows if r["id"] == out["series_tag_id"])
+    assert card["name"] == "Card Comic pages 1-2"
+    assert card["chapter_count"] == 1
+    assert card["page_count"] == 2
+    assert card["artist_name"] == "Card Artist"
+    assert card["cover_thumbnail_url"]
+    assert card["has_gap"] is False
+
+    # artist filter keeps it; a different artist id drops it.
+    assert any(r["id"] == out["series_tag_id"] for r in await svc.list_series(artist_id=artist.id))
+    assert all(r["id"] != out["series_tag_id"] for r in await svc.list_series(artist_id=artist.id + 99999))