feat(store): add append-only login_events table with forced RLS

Stage-0 usage measurement (VISION/ADR-016): summary_actions captures acts
(watch/skip/save) but not reads. A reader who logs in weekly and clicks
nothing is invisible — for a reading product that return is the signal the
gate ("usage in >=2 distinct weeks") is defined on. login_events records
THAT a user was active, append-only, one row per user per active day.

Per-user isolation via the same GUC-keyed FORCE RLS policy as migration 003.
No FK to users (mirrors summary_actions) — the cascade footgun is handled by
DeleteUser in a later commit. Adds an up/down reversibility test and registers
the table in both truncate helpers.

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

internal/adapters/store/migrations/010_login_events.down.sql

@@ -0,0 +1,4 @@
+DROP POLICY IF EXISTS login_events_isolation ON login_events;
+ALTER TABLE login_events NO FORCE ROW LEVEL SECURITY;
+ALTER TABLE login_events DISABLE ROW LEVEL SECURITY;
+DROP TABLE IF EXISTS login_events;

internal/adapters/store/migrations/010_login_events.up.sql

@@ -0,0 +1,33 @@
+-- Migration 010: login_events records THAT a user was active (returned and read)
+-- on a given day — the Stage-0 signal summary_actions misses. summary_actions
+-- captures *acts* (watch/skip/save); a reader who logs in weekly and clicks
+-- nothing is otherwise invisible, yet for a reading product that return IS the
+-- signal the gate ("usage in >=2 distinct weeks", VISION/ADR-016) is defined on.
+--
+-- Append-only: one row per user per active day (the request-path throttle in the
+-- web layer enforces that cadence), never updated. Per-user isolation like every
+-- user-owned table.
+--
+-- NO foreign key to users (mirrors summary_actions, migration 002): user_id is
+-- carried for RLS/scoping but the table is decoupled so a stamp never blocks on a
+-- users row. The cost of that decoupling: the users-row cascade does NOT reach
+-- login_events, so DeleteUser must delete it explicitly (see account.go) — the
+-- exact footgun the summary_actions delete work caught.
+CREATE TABLE login_events (
+    id       UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id  UUID NOT NULL,
+    seen_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- (user_id, seen_at) serves both the per-user-per-day throttle lookup
+-- (seen_at >= start-of-today) and the gate query's per-user week bucketing.
+CREATE INDEX idx_login_events_user_seen ON login_events(user_id, seen_at);
+
+-- RLS: identical GUC-keyed policy/pattern to migration 003. FORCE so the table
+-- owner (tapir, non-superuser in prod) is subject to it; an unset GUC yields NULL
+-- → no rows match → deny-all.
+ALTER TABLE login_events ENABLE ROW LEVEL SECURITY;
+ALTER TABLE login_events FORCE ROW LEVEL SECURITY;
+CREATE POLICY login_events_isolation ON login_events
+    FOR ALL
+    USING (user_id = current_setting('tapir.current_user_id', true)::uuid);