feat: the membership sweep, and the state that makes its failures readable (387 C3)

A daily sweep that walks each platform's roster into `platform_membership`.
Daily because memberships change on a BILLING cycle, not a download cadence.

**`membership_sync` is the part that earns its keep.** Without it three very
different situations are one indistinguishable state — the account subscribes
to nothing, the sweep never ran, the sweep failed — and all three leave zero
rows in `platform_membership`. "You are tracking 12 sources you no longer
subscribe to" is correct in the first case and an invitation to cancel things
the operator is actively paying for in the other two. So C4 gates its
CONCLUSIONS on `last_success_at`, not merely its display, and `roster_is_fresh`
is computed server-side so no caller can forget to.

Two timestamps rather than one: `last_attempt_at` moves every run,
`last_success_at` only on a clean walk. The gap between them is the signal —
a sweep hammering a broken credential every day must not look healthy because
it ran recently, and there is a test for exactly that.

Rejected shortcuts, both tempting: `MAX(platform_membership.last_seen_at)`
cannot tell "synced fine, found nothing" from "never synced"; `task_run` is
worse, since its retention prunes ok rows after 24h and a sweep that last
succeeded three days ago would leave no trace at all.

**The fetch completes before anything is written.** That ordering is the safety
property: a walk that dies mid-pagination writes nothing, so a failure can
never leave a roster half this week's and half last week's. `touch_membership`
never deletes, so a failure cannot empty the roster either — but "intact"
should mean intact, not merely non-empty.

Rule 89's four, each where it actually lives: recovery is "run it again"
(upsert, no deletes); retention is C1's age-out-never-delete, because
disappearing IS the signal; the wall-clock deadline is per-platform and
distinct from the per-REQUEST timeout the client already has (rule 156 — a
paginated roster answering every page slowly-but-within-timeout would never
trip that one and would sit on a worker indefinitely); duration comes from the
existing TaskRun signal plumbing.

**A bug caught in review, not production:** the broad `except Exception` would
have swallowed Celery's SoftTimeLimitExceeded — which is an ORDINARY Exception
subclass, not a BaseException — letting the sweep run past the soft limit into
the hard one, where it is SIGKILLed mid-transaction. A sweep that cannot be
stopped is worse than one that fails. Now re-raised explicitly, with a test
that also asserts SoftTimeLimitExceeded is still an Exception, so the re-raise
cannot quietly become dead code.

Rule 164 is why this ships with UI rather than backend-only: a roster that
never synced must be VISIBLE as such. The card says "never synced" in words and
states no count at all — rendering it as 0 is the precise conflation the whole
step exists to prevent — while a real zero behind a real sync is reported as
zero, because that one IS an answer. Pinned in both directions.

Three independent gates decide whether a platform is swept — registered here,
client exposes `iter_memberships`, credential exists — each silent, so adding
SubscribeStar (D1) is one line and nothing else. A missing credential is not an
error: recording a failure would light up the UI for a feature never enabled.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

alembic/versions/0095_membership_sync.py

@@ -0,0 +1,63 @@
+"""membership_sync — whether the roster actually synced, and when.
+
+Milestone 387, step C3.
+
+`platform_membership` (0091) records what was SEEN. This records whether
+looking happened at all — a different fact, and the one that makes an empty
+roster readable.
+
+Without it, three situations collapse into one: the account subscribes to
+nothing, the sweep never ran, or the sweep failed. All three leave zero rows
+in `platform_membership`. "You are tracking 12 sources you no longer subscribe
+to" is correct in the first case and an invitation to cancel things the
+operator is actively paying for in the other two, which is why C4 gates its
+CONCLUSIONS on `last_success_at` rather than merely displaying it.
+
+Two timestamps rather than one, deliberately: `last_attempt_at` moves every
+run, `last_success_at` only on a clean walk, and the gap between them is what
+lets the UI say "last synced 3 days ago, tried 20 minutes ago, failing".
+
+Revision ID: 0095
+Revises: 0094
+Create Date: 2026-09-11
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0095"
+down_revision: Union[str, None] = "0094"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "membership_sync",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("platform", sa.String(length=64), nullable=False),
+        sa.Column("last_attempt_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("last_success_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("last_count", sa.Integer(), nullable=True),
+        # No CHECK: this carries an exception class name, and the vocabulary is
+        # whatever the client raises — same call as source.error_type.
+        sa.Column("last_error_type", sa.String(length=64), nullable=True),
+        sa.Column("last_error_message", sa.Text(), nullable=True),
+        sa.Column(
+            "updated_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_membership_sync")),
+        # The upsert's conflict target, named explicitly because the service
+        # references it by name in ON CONFLICT.
+        sa.UniqueConstraint("platform", name="uq_membership_sync_platform"),
+    )
+    # No secondary indexes: one row per platform, so every read is a short scan
+    # and an index would be write cost buying nothing (#3301 removed seven of
+    # that shape). Same reasoning as platform_membership in 0091.
+
+
+def downgrade() -> None:
+    op.drop_table("membership_sync")