feat(store): add workout_not_found_at, exclude it from missing-workout queries

A confirmed-404 workout must stop being retried forever, but
workout_raw_json should never be fabricated -- it stays null exactly
as it does for "not yet fetched." workout_not_found_at is the
separate marker ActivitiesMissingWorkout/CountActivitiesMissingWorkout
now check for, mirroring the existing pattern of the other _fetched_at
columns. UpsertActivity's ON CONFLICT clause already never touches
these columns, so the marker persists across every later re-sync.

docs/DATABASE.md

@@ -150,6 +150,14 @@ CREATE TABLE activities (
     -- Genuine raw get_workout_by_id() response, the source used to compute
     -- alignWorkoutTargets. Null when the activity has no workout_id.
     workout_raw_json           TEXT,
+    -- Set when get_workout_by_id returned a definitive HTTP 404 (the
+    -- workout was deleted on Garmin's side after being linked to this
+    -- activity) -- distinct from workout_raw_json staying null for "not yet
+    -- fetched": this activity is excluded from ActivitiesMissingWorkout so
+    -- it stops being retried forever (see
+    -- docs/superpowers/specs/2026-07-27-workout-not-found-design.md).
+    -- workout_raw_json itself is never fabricated; it just stays null.
+    workout_not_found_at       TEXT,
     created_at                 TEXT NOT NULL DEFAULT (datetime('now')),
     updated_at                 TEXT NOT NULL DEFAULT (datetime('now')),
     UNIQUE(user_id, garmin_activity_id)