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>

alembic/versions/0041_series_suggestions.py

@@ -0,0 +1,98 @@
+"""series suggestions: assisted-continuation matcher (FC-6.3)
+
+Revision ID: 0041
+Revises: 0040
+Create Date: 2026-06-07
+
+A confirm-only queue of "this post may continue this series" hints, plus two
+import_settings knobs (enable + score threshold) for the matcher.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0041"
+down_revision: Union[str, None] = "0040"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "series_suggestion",
+        sa.Column("id", sa.Integer, primary_key=True),
+        sa.Column(
+            "post_id",
+            sa.Integer,
+            sa.ForeignKey("post.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "series_tag_id",
+            sa.Integer,
+            sa.ForeignKey("tag.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("score", sa.Float, nullable=False),
+        sa.Column("signals", sa.JSON, nullable=True),
+        sa.Column(
+            "status", sa.String(16), nullable=False, server_default="pending"
+        ),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+        sa.UniqueConstraint(
+            "post_id", "series_tag_id", name="uq_series_suggestion_post_series"
+        ),
+    )
+    op.create_index(
+        "ix_series_suggestion_post_id", "series_suggestion", ["post_id"]
+    )
+    op.create_index(
+        "ix_series_suggestion_series_tag_id",
+        "series_suggestion",
+        ["series_tag_id"],
+    )
+    op.create_index(
+        "ix_series_suggestion_status", "series_suggestion", ["status"]
+    )
+
+    op.add_column(
+        "import_settings",
+        sa.Column(
+            "series_suggest_enabled",
+            sa.Boolean,
+            nullable=False,
+            server_default=sa.true(),
+        ),
+    )
+    op.add_column(
+        "import_settings",
+        sa.Column(
+            "series_suggest_threshold",
+            sa.Float,
+            nullable=False,
+            server_default="0.5",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("import_settings", "series_suggest_threshold")
+    op.drop_column("import_settings", "series_suggest_enabled")
+    op.drop_index("ix_series_suggestion_status", table_name="series_suggestion")
+    op.drop_index(
+        "ix_series_suggestion_series_tag_id", table_name="series_suggestion"
+    )
+    op.drop_index("ix_series_suggestion_post_id", table_name="series_suggestion")
+    op.drop_table("series_suggestion")