feat(patreon): capture full post body via adaptive detail-fetch

The feed endpoint (/api/posts) returns `content` empty for many posts, so post
bodies — their formatting, inline <img>, and external <a href> links — were
never captured (the post showed "(no description)"). Enrich an empty feed body
from the per-post detail endpoint (/api/posts/{id}) before writing the importer
sidecar, memoized by mutating the shared post dict so a multi-image post fetches
detail exactly once and fully-seen posts (no fresh download) pay nothing.
Best-effort by design: a body we can't fetch returns None and never fails the
walk. No-doubling and no-clobber-of-populated-body already hold (post upsert is
keyed on external_post_id; an empty body parses to None and isn't applied).

First slice of milestone #64 (rich post capture + faithful rendering +
external-host downloads). Refs FC #830.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/patreon_downloader.py

@@ -168,10 +168,17 @@ class PatreonDownloader:
         validate: bool = True,
         rate_limit: float = 0.0,
         session: requests.Session | None = None,
+        content_fetcher: Callable[[str], str | None] | None = None,
     ):
         self.images_root = Path(images_root)
         self.cookies_path = str(cookies_path) if cookies_path else None
         self._validate = validate
+        # Best-effort enrichment seam: (post_id) -> full HTML body, or None. The
+        # feed endpoint often omits `content`; the adapter wires this to
+        # PatreonClient.fetch_post_detail_content so the sidecar captures the
+        # real body (formatting + inline <img> + external <a href> links).
+        # None in unit tests / when enrichment isn't wanted.
+        self._content_fetcher = content_fetcher
         # Politeness: seconds to sleep before each actual media download (paces
         # the CDN; honors ImportSettings.download_rate_limit_seconds, the same
         # value gallery-dl used as its between-downloads `sleep`). 0 = no pacing.
@@ -495,6 +502,18 @@ class PatreonDownloader:
         attrs = post.get("attributes") or {}
         title = attrs.get("title")
         content = attrs.get("content")
+        # The feed/list endpoint frequently returns an empty `content`; the full
+        # HTML body (formatting + inline <img> + external <a href> links) only
+        # comes from the per-post detail endpoint. Enrich on first write for this
+        # post and MEMOIZE by mutating the shared `post` dict — so a multi-image
+        # post fetches detail exactly once, and a fully-seen post (no fresh
+        # download → no sidecar write) never pays the extra GET.
+        if (not isinstance(content, str) or not content.strip()) and self._content_fetcher:
+            fetched = self._content_fetcher(str(post.get("id") or ""))
+            if fetched:
+                attrs["content"] = fetched
+                post["attributes"] = attrs
+                content = fetched
         published = attrs.get("published_at")
         url = attrs.get("url")
         data = {