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_page_number_parser.py

@@ -0,0 +1,39 @@
+"""Pure unit tests for the stated-page parser (FC-6.2)."""
+
+from backend.app.services.page_number_parser import parse_page_range
+
+
+def test_explicit_ranges():
+    assert parse_page_range("pages 9-12") == (9, 12)
+    assert parse_page_range("pg 9–12") == (9, 12)        # en-dash
+    assert parse_page_range("pp. 9 - 12") == (9, 12)
+    assert parse_page_range("Chapter 2 (pages 1-4)") == (1, 4)
+
+
+def test_reversed_range_is_normalized():
+    assert parse_page_range("pages 12-9") == (9, 12)
+
+
+def test_single_page():
+    assert parse_page_range("Page 5") == (5, 5)
+    assert parse_page_range("pg 5") == (5, 5)
+
+
+def test_fraction_forms_take_the_numerator():
+    assert parse_page_range("[3/8]") == (3, 3)
+    assert parse_page_range("(3/8)") == (3, 3)
+    assert parse_page_range("page 3 of 8") == (3, 3)
+    assert parse_page_range("pg 3/8") == (3, 3)
+
+
+def test_no_page_keyword_is_none():
+    # Bare numbers / non-page words must not match (false positives are worse).
+    assert parse_page_range("My Comic Part Three") is None
+    assert parse_page_range("Episode 5") is None
+    assert parse_page_range("Released 2024") is None
+    assert parse_page_range("Part 3") is None
+
+
+def test_empty_and_none():
+    assert parse_page_range("") is None
+    assert parse_page_range(None) is None