diag(subscribestar): name the interstitial in the drift error (title + type)

The XHR fix worked (we now get a real 93KB HTML page, not JSON) but cheunart
still drifts — we're being served a full HTML page that isn't the feed. Add
_describe_page(): the drift error now reports the page <title> + which known
interstitial it resembles (cloudflare/bot-challenge, age-gate, login, captcha),
so the next run names the actual cause instead of "markup changed". Strong
suspicion: a Cloudflare challenge (python-requests has no JS; cf_clearance is
UA-locked and our hardcoded UA likely differs from the cookie-capturing browser).

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

backend/app/services/subscribestar_client.py

@@ -85,6 +85,33 @@ _NEXT_PAGE_RE = re.compile(
 # age cookie missing) rather than a real — possibly empty — creator feed.
 _LOGIN_MARKERS = ("/session/new", 'data-role="sign_in"', "age_confirmation_warning")
 
+# An interstitial served INSTEAD of the feed — characterized so a drift error
+# reports the actual cause (bot challenge / age gate / login) rather than a bare
+# "markup changed". (name, substrings to look for, case-insensitive.)
+_INTERSTITIAL_MARKERS = (
+    ("cloudflare/bot-challenge", (
+        "just a moment", "cf-challenge", "challenge-platform", "cf_chl",
+        "attention required", "enable javascript and cookies",
+    )),
+    ("age-gate", (
+        "18 or older", "adult content", "age_confirmation", "i am over",
+        "confirm your age", "must be 18",
+    )),
+    ("login", ("/session/new", 'data-role="sign_in"', "sign in", "log in")),
+    ("captcha", ("g-recaptcha", "hcaptcha", "captcha")),
+)
+
+
+def _describe_page(html: str) -> str:
+    """A short, log-safe description of an unexpected page: its <title> + which
+    known interstitial it resembles (bot challenge / age gate / login / captcha)."""
+    m = re.search(r"<title[^>]*>(.*?)</title>", html, re.IGNORECASE | re.DOTALL)
+    title = unescape(m.group(1).strip())[:120] if m else "(no <title>)"
+    low = html.lower()
+    hits = [name for name, needles in _INTERSTITIAL_MARKERS
+            if any(n.lower() in low for n in needles)]
+    return f"title={title!r}; resembles: {'/'.join(hits) if hits else 'unrecognized'}"
+
 
 class SubscribeStarAPIError(NativeIngestError):
     """Base for native SubscribeStar client failures. status_code / retry_after
@@ -393,16 +420,15 @@ class SubscribeStarClient:
                 # body canary catches systematic emptiness across a populated feed;
                 # here we only raise if the feed container itself is missing.
                 if 'data-role="posts_container-list"' not in html:
-                    # Surface what we actually got (length + a JSON hint) — the
-                    # first live run tripped this because an XHR-flagged GET
-                    # returned a non-HTML body, which is invisible from a bare
-                    # "markup changed?" message.
+                    # Report what we actually got — length + the page's identity —
+                    # so a served interstitial (bot challenge / age gate / login)
+                    # is named instead of mislabeled "markup changed".
                     looks_json = html.lstrip()[:1] in ("{", "[")
                     raise SubscribeStarDriftError(
                         f"SubscribeStar feed for {slug!r} had no posts and no "
                         f"recognizable feed container ({len(html)} bytes"
-                        f"{', looks like JSON' if looks_json else ''}) — "
-                        "markup changed, or the feed wasn't served as a full page?"
+                        f"{', looks like JSON' if looks_json else ''}; "
+                        f"{_describe_page(html)})"
                     )
             for post in posts:
                 post["_base"] = base