feat(series): assisted-continuation matcher + suggestion queue — backend (FC-6.3)

Confirm-only "this post may continue this series" matcher.

- series_suggestion table (post_id, series_tag_id, score, signals jsonb, status
  pending|added|dismissed, UNIQUE(post,series)); migration 0041 + two settings
  knobs (series_suggest_enabled, series_suggest_threshold).
- series_match_service: weighted additive score (title-stem / same-artist /
  page-continuity / shared-distinctive-tags), no single signal gating. The title
  "pattern" is derived on the fly from the post titles already in a series, so it
  sharpens as more are confirmed (no persisted state to drift). Candidates are
  bounded to the post's artist. match_post upserts pending suggestions (UNIQUE +
  on-conflict, respecting prior added/dismissed decisions).
- accept reuses add_post_as_chapter then marks 'added'; dismiss marks 'dismissed'.
- rescan_series_suggestions_task: settings-gated, time-boxed + self-resuming from
  a post-id cursor (maintenance_long lane), like normalize_tags_task.
- API: GET /series/suggestions, POST .../<id>/accept|dismiss, POST .../rescan.
- Settings: enabled + threshold exposed via /settings/import.
- Tests: pure scoring helpers + matcher/accept/dismiss/rescan lifecycle + UNIQUE
  dedup.

Frontend (Suggestions tab + settings card) lands next.

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

backend/app/models/import_settings.py

@@ -64,6 +64,15 @@ class ImportSettings(Base):
         Integer, nullable=False, default=3,
     )
 
+    # FC-6.3 series continuation matcher. enabled gates the rescan; threshold is
+    # the weighted-score cut-off (0..1) above which a pending suggestion is made.
+    series_suggest_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True,
+    )
+    series_suggest_threshold: Mapped[float] = mapped_column(
+        Float, nullable=False, default=0.5,
+    )
+
     @classmethod
     async def load(cls, session) -> ImportSettings:
         """The singleton settings row (id=1), via an async session."""