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/api/settings.py

@@ -25,6 +25,8 @@ _EDITABLE_FIELDS = (
     "download_schedule_default_seconds",
     "download_event_retention_days",
     "download_failure_warning_threshold",
+    "series_suggest_enabled",
+    "series_suggest_threshold",
 )
 
 
@@ -46,6 +48,8 @@ async def get_import_settings():
         "download_schedule_default_seconds": row.download_schedule_default_seconds,
         "download_event_retention_days": row.download_event_retention_days,
         "download_failure_warning_threshold": row.download_failure_warning_threshold,
+        "series_suggest_enabled": row.series_suggest_enabled,
+        "series_suggest_threshold": row.series_suggest_threshold,
     })
 
 
@@ -96,6 +100,19 @@ async def update_import_settings():
         if not isinstance(v, int) or isinstance(v, bool) or v < 1 or v > 100:
             return _bad_int("download_failure_warning_threshold", 1, 100)
 
+    if "series_suggest_enabled" in body and not isinstance(
+        body["series_suggest_enabled"], bool
+    ):
+        return jsonify(
+            {"error": "series_suggest_enabled must be a boolean"}
+        ), 400
+    if "series_suggest_threshold" in body:
+        v = body["series_suggest_threshold"]
+        if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0 or v > 1:
+            return jsonify(
+                {"error": "series_suggest_threshold must be a number in [0, 1]"}
+            ), 400
+
     async with get_session() as session:
         row = await ImportSettings.load(session)
         for field in _EDITABLE_FIELDS:

backend/app/api/tags.py

@@ -8,6 +8,7 @@ from ..extensions import get_session
 from ..models import Tag, TagKind
 from ..models.tag_allowlist import TagAllowlist
 from ..services.bulk_tag_service import BulkTagService
+from ..services.series_match_service import SeriesMatchService
 from ..services.series_service import SeriesError, SeriesService
 from ..services.tag_directory_service import TagDirectoryService
 from ..services.tag_service import (
@@ -653,3 +654,43 @@ async def series_add_post(tag_id: int):
             return _series_err(exc)
         await session.commit()
     return jsonify(out)
+
+
+# ---- suggestion queue (FC-6.3) --------------------------------------------
+
+
+@tags_bp.route("/series/suggestions", methods=["GET"])
+async def series_suggestions_list():
+    async with get_session() as session:
+        rows = await SeriesMatchService(session).list_pending()
+    return jsonify({"suggestions": rows})
+
+
+@tags_bp.route("/series/suggestions/<int:sid>/accept", methods=["POST"])
+async def series_suggestion_accept(sid: int):
+    async with get_session() as session:
+        try:
+            out = await SeriesMatchService(session).accept(sid)
+        except SeriesError as exc:
+            return _series_err(exc)
+        await session.commit()
+    return jsonify(out)
+
+
+@tags_bp.route("/series/suggestions/<int:sid>/dismiss", methods=["POST"])
+async def series_suggestion_dismiss(sid: int):
+    async with get_session() as session:
+        try:
+            await SeriesMatchService(session).dismiss(sid)
+        except SeriesError as exc:
+            return _series_err(exc)
+        await session.commit()
+    return jsonify({"ok": True})
+
+
+@tags_bp.route("/series/suggestions/rescan", methods=["POST"])
+async def series_suggestions_rescan():
+    from ..tasks.admin import rescan_series_suggestions_task
+
+    res = rescan_series_suggestions_task.delay()
+    return jsonify({"task_id": res.id})