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/__init__.py

@@ -20,6 +20,7 @@ from .post import Post
 from .post_attachment import PostAttachment
 from .series_chapter import SeriesChapter
 from .series_page import SeriesPage
+from .series_suggestion import SeriesSuggestion
 from .source import Source
 from .tag import Tag, TagKind, image_tag
 from .tag_alias import TagAlias
@@ -42,6 +43,7 @@ __all__ = [
     "PostAttachment",
     "SeriesChapter",
     "SeriesPage",
+    "SeriesSuggestion",
     "ImageRecord",
     "ImageProvenance",
     "Tag",

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."""

backend/app/models/series_suggestion.py

@@ -0,0 +1,55 @@
+"""SeriesSuggestion — a confirm-only "this post may continue this series" hint.
+
+The matcher (FC-6.3) scores a (post, candidate series) pair from several weighted
+signals and, above the configured threshold, records a pending suggestion. The
+operator confirms (→ the post is added as a chapter) or dismisses it; FC never
+files a post into a series on its own. status is a plain string (no Postgres
+ENUM — see the check-existing-enums lesson): pending | added | dismissed.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import (
+    JSON,
+    DateTime,
+    Float,
+    ForeignKey,
+    Integer,
+    String,
+    UniqueConstraint,
+    func,
+)
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class SeriesSuggestion(Base):
+    __tablename__ = "series_suggestion"
+    __table_args__ = (
+        UniqueConstraint(
+            "post_id", "series_tag_id", name="uq_series_suggestion_post_series"
+        ),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    post_id: Mapped[int] = mapped_column(
+        ForeignKey("post.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    series_tag_id: Mapped[int] = mapped_column(
+        ForeignKey("tag.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    score: Mapped[float] = mapped_column(Float, nullable=False)
+    signals: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    status: Mapped[str] = mapped_column(
+        String(16), nullable=False, server_default="pending", index=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        server_default=func.now(),
+        onupdate=func.now(),
+    )