fix(subscribestar): inject 18_plus_agreement_generic age cookie to bypass server gate

Operator-flagged 2026-05-27: subscribestar source check aborted with
`AbortExtraction: HTTP redirect to .../age_confirmation_warning`. The
captured `_personalization_id` cookie in the browser-stored file had
expired (annual rotation), and the user could not realistically refresh
it: SubscribeStar's frontend JS uses localStorage to suppress the
age-confirmation popup once dismissed, so a logged-in revisit doesn't
re-show the popup and the server-side cookie is never re-issued.

gallery-dl's own login flow (which FC doesn't exercise — cookies come
from the extension instead) sidesteps this by manually setting
`18_plus_agreement_generic=true` on `.subscribestar.adult`. The server
accepts that as the age-confirmation marker.

`credential_service._augment_cookies(platform, netscape)` mirrors that
behavior: when the materialized cookies file is for subscribestar and
the age cookie isn't already present, append a synthetic line for
`.subscribestar.adult` with name=`18_plus_agreement_generic` value=`true`
and a far-future expiry. No-op for other platforms; no-op if the cookie
is already present (idempotent for manual pastes / extension captures
that happen to include it).

Three new tests pin: (a) injection fires for subscribestar, preserves
existing cookies; (b) idempotent when already present (no double
injection); (c) does NOT fire for non-subscribestar platforms (Patreon
etc. don't get a foreign-domain cookie).

Not a curator handling bug per se — the extension faithfully captured
what the browser had. This is mirroring a documented gallery-dl
workaround so the cookies-via-extension auth path doesn't degrade as the
server-side cookie expires.

tests/test_credential_service.py

@@ -110,6 +110,57 @@ async def test_get_cookies_path_none_for_token_kind(db, crypto, tmp_path):
     assert await svc.get_cookies_path("discord") is None
 
 
+@pytest.mark.asyncio
+async def test_get_cookies_path_subscribestar_injects_age_cookie(db, crypto, tmp_path):
+    """SubscribeStar's server gates artist pages behind a _personalization_id
+    cookie; the browser-stored cookie expires annually and can't be easily
+    refreshed (the JS age popup is suppressed by localStorage). Mirror
+    gallery-dl's own login-flow workaround by injecting
+    `18_plus_agreement_generic=true` on `.subscribestar.adult` whenever
+    cookies for subscribestar are materialized."""
+    netscape_in = (
+        "# Netscape HTTP Cookie File\n"
+        ".subscribestar.adult\tTRUE\t/\tTRUE\t1700000000\tsession_id\txyz\n"
+    )
+    svc = CredentialService(db, crypto, cookies_dir=tmp_path / "cookies")
+    await svc.upsert(platform="subscribestar", credential_type="cookies", data=netscape_in)
+    path = await svc.get_cookies_path("subscribestar")
+    contents = path.read_text()
+    assert "18_plus_agreement_generic\ttrue" in contents
+    assert ".subscribestar.adult" in contents
+    # Original session_id cookie preserved.
+    assert "session_id\txyz" in contents
+
+
+@pytest.mark.asyncio
+async def test_get_cookies_path_subscribestar_idempotent_when_present(db, crypto, tmp_path):
+    """If the operator's captured cookies ALREADY contain the age cookie
+    (e.g. a manual paste, or a re-login), don't double-inject."""
+    netscape_in = (
+        "# Netscape HTTP Cookie File\n"
+        ".subscribestar.adult\tTRUE\t/\tTRUE\t1700000000\t18_plus_agreement_generic\ttrue\n"
+        ".subscribestar.adult\tTRUE\t/\tTRUE\t1700000000\tsession_id\txyz\n"
+    )
+    svc = CredentialService(db, crypto, cookies_dir=tmp_path / "cookies")
+    await svc.upsert(platform="subscribestar", credential_type="cookies", data=netscape_in)
+    path = await svc.get_cookies_path("subscribestar")
+    contents = path.read_text()
+    assert contents.count("18_plus_agreement_generic") == 1
+
+
+@pytest.mark.asyncio
+async def test_get_cookies_path_non_subscribestar_unchanged(db, crypto, tmp_path):
+    """The age-cookie injection MUST NOT fire for non-subscribestar
+    platforms — Patreon/HF/etc. don't need it and shouldn't carry a
+    foreign-domain cookie in their cookies.txt."""
+    svc = CredentialService(db, crypto, cookies_dir=tmp_path / "cookies")
+    await svc.upsert(platform="patreon", credential_type="cookies", data=_NETSCAPE)
+    path = await svc.get_cookies_path("patreon")
+    contents = path.read_text()
+    assert "18_plus_agreement_generic" not in contents
+    assert "subscribestar" not in contents
+
+
 @pytest.mark.asyncio
 async def test_get_token_decrypts(db, crypto):
     svc = CredentialService(db, crypto)