fix(subscribestar): match gallery-dl's generic post delimiter (live-feed drift)

The native client split the feed on `<div class="post is-shown`, but `is-shown`
is added by SubscribeStar's infinite-scroll JS when a post scrolls into view —
present in a browser-SAVED page (what the Step-0 characterization used) but
ABSENT from the raw server HTML we and gallery-dl actually fetch. So the live
feed (cheunart) parsed to zero posts and raised a false SubscribeStarDriftError.

Align with gallery-dl's proven `_pagination`: split on the generic
`<div class="post ` (trailing space rules out the hyphenated post-content/
post-date/post-body siblings). Also mirror gallery-dl's redirect-based gating
detection (/verify_subscriber, /age_confirmation_warning => auth, not drift).

Regression tests: raw server markup without is-shown now parses; an age-wall
redirect raises SubscribeStarAuthError.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/subscribestar_client.py

@@ -68,7 +68,16 @@ _LOADMORE_HEADERS = {
 # A post block opens with this wrapper; we slice the page between consecutive
 # occurrences (regex can't match the balanced close of nested divs, so chunk-
 # per-post is the robust approach gallery-dl also uses).
-_POST_OPEN = '<div class="post is-shown'
+#
+# Delimiter is the GENERIC `<div class="post ` (trailing space — matches the post
+# container regardless of the classes that follow), exactly what gallery-dl's
+# `_pagination` splits on. We previously keyed on `<div class="post is-shown`,
+# but `is-shown` is added by SubscribeStar's infinite-scroll JS when a post
+# scrolls into view — it's present in a browser-SAVED page but ABSENT from the
+# raw server HTML we (and gallery-dl) actually fetch, so the raw feed parsed to
+# zero posts → false drift (cheunart, 2026-06-17). The space rules out the
+# hyphenated siblings (`post-content`/`post-date`/`post-body`/`post-uploads`).
+_POST_OPEN = '<div class="post '
 _POST_ID_RE = re.compile(r'data-id="(\d+)"')
 _POST_DATE_RE = re.compile(r'<div class="post-date">([^<]+)</div>')
 # The body: inner of `.post-content .trix-content`. Non-greedy to the LAST
@@ -216,6 +225,19 @@ class SubscribeStarClient:
                 time.sleep(delay)
                 continue
             break
+        # gallery-dl's own gating signal: SubscribeStar 302-redirects an
+        # unauthenticated / age-unconfirmed request to /verify_subscriber or
+        # /age_confirmation_warning (lands as a 200 on that URL). Treat it as auth,
+        # not drift — the fix is to rotate cookies / set the age cookie.
+        if resp.history and (
+            "/verify_subscriber" in resp.url
+            or "/age_confirmation_warning" in resp.url
+        ):
+            raise SubscribeStarAuthError(
+                f"SubscribeStar redirected to {resp.url} — auth/age wall "
+                f"(rotate cookies or set the 18+ age cookie; requested {url})",
+                status_code=resp.status_code,
+            )
         if resp.status_code in (401, 403):
             raise SubscribeStarAuthError(
                 f"SubscribeStar returned HTTP {resp.status_code} — auth rejected "