fix(sidecar): correct external_post_id + post_url derivation for non-Patreon platforms

Audit of one sample sidecar per platform on the operator's
/mnt/Data/Patreon/ archive surfaced three parser bugs that have been
silently corrupting non-Patreon Posts since FC-3 shipped:

1. SubscribeStar `id` vs `post_id` confusion. gallery-dl puts the
   per-attachment id in `id` (e.g. 711509) and the actual post id in
   `post_id` (e.g. 360360). FC's external_post_id chain had `id`
   winning, so every multi-image SubscribeStar post was fragmented into
   N Post rows in the database. Reorder the chain to
   `("post_id", "id", "index", "message_id")` — Patreon/Pixiv (no
   `post_id`), HF (uses `index`), Discord (uses `message_id`) all
   unaffected.

2. Discord `message` field not captured. Discord posts put the body in
   `message`, not `content`. Append it to the description fallback chain
   `("content", "description", "caption", "message")`.

3. post_url is the file URL on SubscribeStar/Pixiv/HF/Discord. New
   `_derive_post_url(platform, data)` helper synthesizes proper
   permalinks from per-platform fields:
     subscribestar → https://www.subscribestar.com/posts/<post_id>
     pixiv         → https://www.pixiv.net/artworks/<id>
     hentaifoundry → https://www.hentai-foundry.com/pictures/user/<user>/<index>
     discord       → https://discord.com/channels/<server>/<channel>/<message>
   Patreon's bare `url` IS a real permalink and is used as-is. For the
   four file-URL platforms, the bare `url` is NEVER trusted: derive or
   return None rather than persist a CDN URL.

Tests:
- `test_parse_core_fields_and_id_priority` flipped to assert post_id
  wins over id.
- New `test_parse_id_used_when_no_post_id` covers the Patreon real
  shape.
- New `test_parse_message_used_as_description_fallback` covers Discord
  bodies.
- Five new tests cover per-platform post_url derivation
  (SubscribeStar/Pixiv/HF/Discord/Patreon-untouched + missing-fields →
  None).

Cleanup migration alembic 0025_fix_subscribestar_post_ids:
- For each SubscribeStar Post: find a related ImageRecord.path, walk to
  its sidecar JSON, read `post_id`, overwrite Post.external_post_id +
  post_url with the corrected values.
- After all updates, every group of Posts under one source sharing the
  same NEW external_post_id is a fragment-set — merge to a canonical
  row using the same ImageProvenance pre-delete + repoint dance as
  alembic 0022 (banked pattern).
- Pure-SQL backfill of Pixiv post_url: replace any `i.pximg.net`-shape
  url with the derived `/artworks/<id>` permalink.
- HF and Discord post_url backfills skipped — HF would need the `user`
  field (not stored on Post), Discord needs server/channel triple.
  Both will be corrected by a deep-scan re-applying sidecars through
  the new parser.

Idempotent: re-running on already-corrected data is a no-op.

backend/app/utils/sidecar.py

@@ -111,8 +111,16 @@ def parse_sidecar(data: dict) -> SidecarData:
     cat = data.get("category")
     platform = cat if isinstance(cat, str) and cat.strip() else None
 
+    # external_post_id lookup order: post_id MUST come before id.
+    # SubscribeStar gallery-dl writes the per-attachment id in `id`
+    # (e.g. 711509) and the actual post id in `post_id` (e.g. 360360);
+    # picking `id` first fragments every multi-image subscribestar post
+    # into N distinct Post rows in FC. Patreon/Pixiv have no `post_id`
+    # so `id` still wins for them; HF uses `index`, Discord uses
+    # `message_id` — all reached via the remaining chain entries.
+    # Operator-flagged 2026-05-27 during the sidecar audit.
     external_post_id = None
-    for k in ("id", "post_id", "index", "message_id"):
+    for k in ("post_id", "id", "index", "message_id"):
         v = data.get(k)
         if isinstance(v, bool):
             continue
@@ -138,7 +146,11 @@ def parse_sidecar(data: dict) -> SidecarData:
             if post_date is not None:
                 break
 
-    description = _first_str(data, ("content", "description", "caption"))
+    # `message` is Discord gallery-dl's body field (no `content`); added
+    # 2026-05-27 to the description fallback chain.
+    description = _first_str(
+        data, ("content", "description", "caption", "message"),
+    )
 
     # SubscribeStar posts always write `title: ""` and put the leading
     # sentence inside `content` (confirmed against the operator's
@@ -150,13 +162,60 @@ def parse_sidecar(data: dict) -> SidecarData:
     if post_title is None and description:
         post_title = _first_line_text(description)
 
+    # post_url derivation: SubscribeStar/Pixiv/HF/Discord put the FILE
+    # download URL in `url`, not a post permalink. Synthesize the
+    # permalink from per-platform fields when possible. Patreon's `url`
+    # IS a permalink and is used as-is. For the four file-URL platforms,
+    # the bare `url` is NEVER trusted — derive or return None rather
+    # than persist a CDN URL in post.post_url.
+    if platform in _DERIVED_URL_PLATFORMS:
+        post_url = _derive_post_url(platform, data)
+    else:
+        post_url = _first_str(data, ("url", "post_url"))
+
     return SidecarData(
         platform=platform,
         external_post_id=external_post_id,
-        post_url=_first_str(data, ("url", "post_url")),
+        post_url=post_url,
         post_title=post_title,
         description=description,
         attachment_count=attachment_count,
         post_date=post_date,
         raw=data,
     )
+
+
+_DERIVED_URL_PLATFORMS = frozenset({
+    "subscribestar", "pixiv", "hentaifoundry", "discord",
+})
+
+
+def _derive_post_url(platform: str, data: dict) -> str | None:
+    """Synthesize the post-permalink URL from per-platform metadata.
+
+    gallery-dl writes the file-download URL in `url` for these four
+    platforms; we need a real permalink for the PostCard "open original"
+    button. Returns None if the platform-specific fields are missing
+    (rare in well-formed sidecars but defensive).
+    """
+    if platform == "subscribestar":
+        pid = data.get("post_id")
+        if isinstance(pid, (str, int)) and str(pid).strip():
+            return f"https://www.subscribestar.com/posts/{pid}"
+    elif platform == "pixiv":
+        pid = data.get("id")
+        if isinstance(pid, (str, int)) and not isinstance(pid, bool) and str(pid).strip():
+            return f"https://www.pixiv.net/artworks/{pid}"
+    elif platform == "hentaifoundry":
+        user = _first_str(data, ("user", "artist"))
+        idx = data.get("index")
+        if user and isinstance(idx, (str, int)) and not isinstance(idx, bool) and str(idx).strip():
+            return f"https://www.hentai-foundry.com/pictures/user/{user}/{idx}"
+    elif platform == "discord":
+        sid = data.get("server_id")
+        cid = data.get("channel_id")
+        mid = data.get("message_id")
+        if all(isinstance(v, (str, int)) and not isinstance(v, bool) and str(v).strip()
+               for v in (sid, cid, mid)):
+            return f"https://discord.com/channels/{sid}/{cid}/{mid}"
+    return None