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_ingester.py

@@ -112,7 +112,10 @@ class PatreonIngester(Ingester):
             downloader
             if downloader is not None
             else PatreonDownloader(
-                self.images_root, cookies_path, validate=validate, rate_limit=rate_limit
+                self.images_root, cookies_path, validate=validate, rate_limit=rate_limit,
+                # Enrich empty feed bodies from the per-post detail endpoint, via
+                # the SAME client (shares its cookie session + request pacing).
+                content_fetcher=resolved_client.fetch_post_detail_content,
             )
         )
         super().__init__(