feat(model): nullable Post.source_id + denormalized Post.artist_id; retire sidecar synthetics

Operator-asked 2026-06-01 after the Dymkens orphan investigation
(Scribe plan #540). The pre-2030 sidecar-synthetic Source pattern
(`sidecar:<platform>:<slug>` enabled=false rows) existed solely to
satisfy `Post.source_id NOT NULL`, and leaked into the Subscriptions
UI as phantom subscriptions. Now the data model says what's true:
filesystem-imported content with no live subscription has NULL
source_id, full stop.

## Schema (alembic 0030)

- `post.artist_id` — NEW NOT NULL FK to artist (CASCADE). Backfilled
  from source.artist_id in the migration. Indexed for the artist-filter
  queries.
- `post.source_id` — NOT NULL → nullable; FK ondelete CASCADE → SET
  NULL. Deleting a Source detaches its Posts instead of destroying
  archived content (subscription ends, archive stays).
- `image_provenance.source_id` — same nullable + SET NULL.
- Partial unique index `uq_post_artist_external_id_null_source` on
  (artist_id, external_post_id) WHERE source_id IS NULL — guards
  filesystem-import dedup since the existing source-bound unique
  ignores NULLs (Postgres treats NULL != NULL).
- Sidecar synthetic Sources deleted: NULL out FKs in post,
  image_provenance first, then DELETE FROM source WHERE url LIKE
  'sidecar:%'. The Dymkens cleanup.

## Model + service changes

- `Post.source_id` → `Mapped[int | None]`; new `Post.artist_id`
  denormalized.
- `ImageProvenance.source_id` → `Mapped[int | None]`.
- Importer: `_source_for_sidecar` (synthetic-creating) →
  `_lookup_source_for_sidecar` (returns None when no subscription).
  `_find_or_create_post` takes required `artist_id`; matches on
  (source_id, external_post_id) for source-bound posts or
  (artist_id, external_post_id) for NULL-source posts.
- Service queries switched off the Source detour to use Post.artist_id
  directly: post_feed_service.scroll/around/get_post (LEFT JOIN to
  Source so NULL-source posts surface); artist_service date_row/
  activity/post_count; provenance_service.for_image/for_post (LEFT
  JOIN); gallery_service._provenance_exists_where_artist via
  Post.artist_id instead of ImageProvenance.source_id → Source.
- `_to_dict` and provenance dict-builders emit `"source": null` for
  NULL-source rows.

## Frontend

- `ProvenancePanel.vue` + `PostCard.vue`: render `e.source?.platform
  ?? 'filesystem import'` so NULL-source posts get a clear
  "filesystem import" affordance instead of a NaN crash.

## Tests

- `test_importer_upsert_helpers`: removed the four synthetic-anchor
  tests; added `_find_or_create_post_idempotent_with_null_source`
  (dedup via the partial unique index) and
  `_lookup_source_for_sidecar_returns_*` (existing-subscription +
  none cases). The existing `_find_or_create_post_idempotent` now
  also passes `artist_id` and asserts it.
- 8 other test files updated: every direct `Post(...)` construction
  gains `artist_id=<artist>.id`. The `_seed_post` helper in
  `test_post_feed_service` looks up artist_id from the source row so
  callsites stay one-arg.

## Verification on deploy

After alembic 0030 runs:
- `SELECT COUNT(*) FROM source WHERE url LIKE 'sidecar:%'` → 0.
- `SELECT COUNT(*) FROM post WHERE source_id IS NULL` → count of
  filesystem-imported posts (Dymkens + any other historical).
- Every `post.artist_id` non-null; consistent with source.artist_id
  for source-bound rows.
- Subscriptions tab: no Dymkens phantom row.
- Artist detail → Posts/Gallery: Dymkens's content still reachable
  via Post.artist_id.
- Provenance panel renders "filesystem import" chip for NULL-source
  posts; PostCard same.

## Out of scope

- UI to manage/delete orphan NULL-source Posts. Data model is right;
  UI follows if operator wants it.

backend/app/services/post_feed_service.py

@@ -69,13 +69,19 @@ class PostFeedService:
             raise ValueError("direction must be 'older' or 'newer'")
 
         sort_key = _sort_key()
+        # Artist via the denormalized Post.artist_id (alembic 0030);
+        # Source via LEFT JOIN since post.source_id can now be NULL for
+        # filesystem-imported posts with no live subscription. A
+        # platform= filter implicitly excludes NULL-source posts (they
+        # have no platform); an artist_id= filter still surfaces them
+        # because Post.artist_id is always set.
         stmt = (
             select(Post, Artist, Source)
-            .join(Source, Post.source_id == Source.id)
-            .join(Artist, Source.artist_id == Artist.id)
+            .join(Artist, Post.artist_id == Artist.id)
+            .outerjoin(Source, Post.source_id == Source.id)
         )
         if artist_id is not None:
-            stmt = stmt.where(Source.artist_id == artist_id)
+            stmt = stmt.where(Post.artist_id == artist_id)
         if platform is not None:
             stmt = stmt.where(Source.platform == platform)
         if cursor:
@@ -135,8 +141,8 @@ class PostFeedService:
         cursor for each end. Returns None if the post doesn't exist."""
         anchor = (await self.session.execute(
             select(Post, Artist, Source)
-            .join(Source, Post.source_id == Source.id)
-            .join(Artist, Source.artist_id == Artist.id)
+            .join(Artist, Post.artist_id == Artist.id)
+            .outerjoin(Source, Post.source_id == Source.id)
             .where(Post.id == post_id)
         )).one_or_none()
         if anchor is None:
@@ -168,8 +174,8 @@ class PostFeedService:
     async def get_post(self, post_id: int) -> dict | None:
         row = (await self.session.execute(
             select(Post, Artist, Source)
-            .join(Source, Post.source_id == Source.id)
-            .join(Artist, Source.artist_id == Artist.id)
+            .join(Artist, Post.artist_id == Artist.id)
+            .outerjoin(Source, Post.source_id == Source.id)
             .where(Post.id == post_id)
         )).one_or_none()
         if row is None:
@@ -260,7 +266,7 @@ class PostFeedService:
         return out
 
     def _to_dict(
-        self, post: Post, artist: Artist, source: Source,
+        self, post: Post, artist: Artist, source: Source | None,
         thumbs_map: dict, atts_map: dict,
     ) -> dict:
         plain_full = html_to_plain(post.description) if post.description else None
@@ -269,6 +275,9 @@ class PostFeedService:
         else:
             description_plain, truncated = truncate_at_word(plain_full, DESCRIPTION_LIMIT)
         thumbs_entry = thumbs_map.get(post.id, {"thumbs": [], "more": 0})
+        # `source` is null for filesystem-imported posts with no live
+        # subscription (alembic 0030). Frontend renders that as a
+        # "filesystem import" affordance instead of a platform chip.
         return {
             "id": post.id,
             "external_post_id": post.external_post_id,
@@ -279,7 +288,10 @@ class PostFeedService:
             "description_plain": description_plain,
             "description_truncated": truncated,
             "artist": {"id": artist.id, "name": artist.name, "slug": artist.slug},
-            "source": {"id": source.id, "platform": source.platform},
+            "source": (
+                {"id": source.id, "platform": source.platform}
+                if source is not None else None
+            ),
             "thumbnails": thumbs_entry["thumbs"],
             "thumbnails_more": thumbs_entry["more"],
             "attachments": atts_map.get(post.id, []),