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_client.py

@@ -562,6 +562,49 @@ class PatreonClient:
                 return
             current_cursor = next_cursor
 
+    # -- detail (full body enrichment) -------------------------------------
+
+    def fetch_post_detail_content(self, post_id: str) -> str | None:
+        """Best-effort fetch of a post's full HTML `content` from the per-post
+        DETAIL endpoint (`/api/posts/{id}`).
+
+        The feed/list endpoint (`/api/posts`) frequently returns `content` as
+        null even though we request it — the full body (its formatting, inline
+        `<img>`, and external `<a href>` links) only comes back from the
+        single-post detail resource. The downloader calls this to enrich a post
+        whose feed body was empty before writing the importer sidecar.
+
+        Best-effort BY DESIGN: the media download is the primary job, so a body
+        we can't fetch must never fail the walk. Every failure path (no id,
+        transport error, non-200, non-JSON, missing/empty content) returns None
+        rather than raising — distinct from the loud drift/auth raises on the
+        feed path, which gate real downloads.
+        """
+        if not post_id:
+            return None
+        if self._request_sleep > 0:
+            time.sleep(self._request_sleep)  # pace the API endpoint (plan #703)
+        url = f"{_POSTS_URL}/{post_id}"
+        params = {"fields[post]": "content", "json-api-version": "1.0"}
+        try:
+            resp = self._session.get(url, params=params, timeout=_TIMEOUT_SECONDS)
+        except requests.RequestException as exc:
+            log.warning("Patreon post-detail fetch failed (post %s): %s", post_id, exc)
+            return None
+        if resp.status_code != 200:
+            log.warning(
+                "Patreon post-detail fetch HTTP %s (post %s)", resp.status_code, post_id
+            )
+            return None
+        try:
+            payload = resp.json()
+        except ValueError:
+            return None
+        data = payload.get("data") if isinstance(payload, dict) else None
+        attrs = data.get("attributes") if isinstance(data, dict) else None
+        content = attrs.get("content") if isinstance(attrs, dict) else None
+        return content if isinstance(content, str) and content.strip() else None
+
     # -- verify ------------------------------------------------------------
 
     def verify_auth(self, campaign_id: str) -> tuple[bool | None, str]: