Index the event-tier columns the sweeps, event log and retention scan (closes #314)

The per-webhook event databases had no secondary indexes, so startup
recovery, the retry and pending sweeps, the queue-depth sampler, the
event log and retention each read whole tables. Indexes declared in
the GORM model tags now serve them, and AutoMigrate adds them to new
and existing databases alike.

Each index also covers deleted_at: GORM adds deleted_at IS NULL to
these queries, and SQLite, with no table statistics, otherwise
prefers the existing deleted_at index. A test checks SQLite's plan
for each statement as GORM builds it.

Rule suppressed: lll on the three event-tier model structs, whose
struct tags cannot wrap.
The resubmitted_from_id scan is left to
#325.

Model: opus-4-8 (implementation); opus-5-5 (rework)

README.md

@@ -1759,6 +1759,28 @@ retries) is individually logged for full observability.
 
 **Relations:** Belongs to Delivery.
 
+#### Event-tier indexes
+
+These indexes on the per-webhook event databases are declared in the model
+tags, so `AutoMigrate` creates them on a fresh and on an existing database:
+
+| Table              | Columns                     | Serves |
+| ------------------ | --------------------------- | ------ |
+| `deliveries`       | `status`, `deleted_at`      | Startup recovery, the retry and pending sweeps every 60 seconds and the queue-depth sampler every 30 seconds, which select deliveries by status |
+| `deliveries`       | `event_id`, `deleted_at`    | The event log, which loads each event's deliveries, and retention, which selects and deletes the deliveries of expired events |
+| `delivery_results` | `delivery_id`, `deleted_at` | The event log, which loads the attempts of a page's deliveries, and retention, which deletes the attempts of expired events |
+| `events`           | `deleted_at`, `created_at`  | Retention, which selects expired events by age |
+| `events`           | `created_at`                | Retention's delete of the expired events themselves |
+
+GORM's soft delete adds `deleted_at IS NULL` to these queries; retention's
+deletes leave it out, but their lookups of expired rows keep it. SQLite keeps
+no statistics on these tables, and without them it rates the `deleted_at`
+index, which every live row matches, above an index on a column matched
+against several values or compared with `<`. So every index but the last also
+covers `deleted_at`. It comes second, so that retention's deletes can use the
+index without it, except in `events`, where `created_at` is compared with `<`
+and SQLite narrows by a `<` only on the last column it uses.
+
 #### Common Fields
 
 Every entity except `Setting` includes these fields from `BaseModel`.