feat(patreon): resolve the creator's campaign from a single-post URL

A source URL like https://www.patreon.com/posts/mimic-in-dungeon-158372536 is a
single-post permalink, not a creator page — the resolver grabbed "posts" as the
vanity and failed (operator-flagged 2026-06-07). Add a resolution path: extract
the trailing post id and follow it to the owning campaign via the Patreon post
API (/api/posts/<id>?include=campaign), so pasting any post URL subscribes to
that creator's whole feed. `posts/` is excluded from the vanity regex so it
can't masquerade as a creator slug.

Resolution order is now: cached override → id: URL → /posts/<id> → vanity
(campaigns API + creator-page scrape). Tests cover the post→campaign resolve
and that /posts/ URLs aren't treated as vanities.

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

tests/test_patreon_resolver.py

@@ -183,3 +183,35 @@ def test_extract_vanity_handles_all_path_prefixes():
     assert extract_vanity("https://patreon.com/cw/Atole") == "Atole"
     # id: URLs are NOT vanities (handled by the id path).
     assert extract_vanity("https://www.patreon.com/id:123") is None
+    # /posts/ permalinks are NOT vanities (handled by the post path) — must not
+    # masquerade as a creator slug named "posts".
+    assert extract_vanity(
+        "https://www.patreon.com/posts/mimic-in-dungeon-158372536"
+    ) is None
+
+
+@pytest.mark.asyncio
+async def test_for_source_resolves_post_url_to_campaign():
+    """A /posts/<slug>-<id> permalink resolves to the OWNING creator's campaign
+    (a subscription walks the whole feed, not one post)."""
+    post_resp = MagicMock()
+    post_resp.status_code = 200
+    post_resp.json.return_value = {
+        "data": {
+            "id": "158372536", "type": "post",
+            "relationships": {"campaign": {"data": {"id": "9988", "type": "campaign"}}},
+        }
+    }
+    calls = []
+
+    def _get(url, **kwargs):
+        calls.append(url)
+        return post_resp
+
+    with patch("backend.app.services.patreon_resolver.requests.get", side_effect=_get):
+        cid, resolved = await resolve_campaign_id_for_source(
+            "https://www.patreon.com/posts/mimic-in-dungeon-158372536", None, {},
+        )
+    assert cid == "9988"
+    assert resolved == "9988"  # newly resolved → caller caches it
+    assert calls and "/api/posts/158372536" in calls[0]