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.

tests/test_post_feed_service.py

@@ -7,6 +7,7 @@ boundary, and the thumbnails/attachments composition.
 from datetime import UTC, datetime, timedelta
 
 import pytest
+from sqlalchemy import select
 
 from backend.app.models import (
     Artist,
@@ -59,8 +60,13 @@ async def _seed_post(
     db, source_id: int, *, external_id: str,
     post_date=None, downloaded_at=None, title=None, description=None,
 ):
+    # Post.artist_id is NOT NULL since alembic 0030; look it up from
+    # the source so callsites don't have to thread the artist through.
+    artist_id = (await db.execute(
+        select(Source.artist_id).where(Source.id == source_id)
+    )).scalar_one()
     p = Post(
-        source_id=source_id, external_post_id=external_id,
+        source_id=source_id, artist_id=artist_id, external_post_id=external_id,
         post_title=title, post_date=post_date,
         description=description,
     )