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/services/page_number_parser.py

@@ -0,0 +1,41 @@
+"""Parse a stated page number/range out of a post's title/description (FC-6.2).
+
+Artists often state where an installment sits in a series — "pages 9-12",
+"Page 5", "[3/8]". We use that to order chapters and flag missing-page gaps.
+This is best-effort: a confident match wins, otherwise we return None and the
+caller falls back to capture/post-date order. Keep it conservative — a wrong
+page number is worse than no page number — so matches require an explicit
+page keyword (page/pg/pp) or a bracketed N/M fraction, never a bare number.
+
+Supported forms (case-insensitive):
+  range    "pages 9-12", "pg 9–12", "pp. 9 - 12"        -> (9, 12)
+  fraction "page 3 of 8", "pg 3/8", "[3/8]", "(3/8)"     -> (3, 3)
+  single   "page 5", "pg 5", "pp 5"                       -> (5, 5)
+"""
+
+import re
+
+# Page keyword: page/pages/pg/pgs/pp/pp.  (NOT a bare "p" — too many false hits.)
+_KW = r"(?:pages?|pgs?|pp\.?)"
+_DASH = r"[-–—]"
+
+_RANGE = re.compile(rf"\b{_KW}\s*(\d{{1,4}})\s*{_DASH}\s*(\d{{1,4}})", re.I)
+_OF = re.compile(rf"\b{_KW}\s*(\d{{1,4}})\s*(?:of|/)\s*\d{{1,4}}\b", re.I)
+_BRACKET = re.compile(r"[\[(]\s*(\d{1,4})\s*/\s*\d{1,4}\s*[\])]")
+_SINGLE = re.compile(rf"\b{_KW}\s*(\d{{1,4}})\b", re.I)
+
+
+def parse_page_range(text: str | None) -> tuple[int, int] | None:
+    """Return (start, end) or None. start <= end; a single page yields (n, n)."""
+    if not text:
+        return None
+    m = _RANGE.search(text)
+    if m:
+        a, b = int(m.group(1)), int(m.group(2))
+        return (a, b) if a <= b else (b, a)
+    for rx in (_OF, _BRACKET, _SINGLE):
+        m = rx.search(text)
+        if m:
+            n = int(m.group(1))
+            return (n, n)
+    return None