Make SQLite durable under concurrent readers and stop re-delivering stranded webhooks (closes #256)

An operator running `sqlite3 <db> .dump` against their own per-webhook
database wedged it: inbound webhooks rejected with HTTP 500, delivered
webhooks stranded at `pending`, and every one of them POSTed a second
time on the next restart while the event log recorded a single attempt.

Durability. Every SQLite file — main, per-webhook, and archive — now
opens through one path, `internal/database/sqlite_open.go`, in WAL
journal mode with a 10-second busy timeout, `BEGIN IMMEDIATE`
transactions, and a bounded connection pool. WAL is what stops a reader
blocking writers at all. `_txlock=immediate` is what stops a `COMMIT`
failing while its transaction stays open on a pooled connection, which
is how four `database is locked` errors became 593 `cannot start a
transaction within a transaction`. `cache=shared` is gone, because
under it an in-process conflict is SQLITE_LOCKED, which the busy
handler does not retry. The busy timeout is applied before
journal_mode: the driver runs DSN pragmas in order on every new
connection, and `PRAGMA journal_mode` takes a lock, so the reverse
order leaves the one pragma that can block uncovered by the handler
meant to cover it.

Eligibility. `internal/delivery/inflight.go` holds the set of
deliveries the engine owns — taken when a task is queued, when a
target schedules a retry, and by every recovery path before it
re-dispatches; dropped when the worker that ran the task returns.
Recovery and both sweep arms re-dispatch only what the set does not
hold. Nothing decides that from a row's age: a delivery waiting in a
10000-deep channel is arbitrarily old and perfectly healthy, and
reasoning from age re-sends it. `takeForRedispatch` is the single gate
every re-dispatch goes through — ownership first, then a conditional
update confirming the row is still in the status the batch read.

Bookkeeping. `recordResult` and `updateDeliveryStatus` return their
errors instead of logging and dropping them, and a caller whose
bookkeeping write failed writes nothing at all: the delivery keeps
whichever non-terminal status it already held, and the sweeps recover
it. Every recovery path — pending and retrying alike — first settles
any delivery that already holds a successful `DeliveryResult` rather
than sending it again. Recovery continues each delivery's own attempt
numbering instead of restarting at 1. The sweep gains a
`pending`-with-age-bound arm, so a stranded delivery no longer waits
for a restart.

Docs. WAL produces `-wal`/`-shm` sidecars, so the backup and restore
procedures in README.md are corrected against measurement: both
documented procedures were re-run against a live instance, a `-wal`
left by a crash carries data the `.db` alone does not, and an archive
file normally holds its rows in a `-wal` rather than in the `.db`.

internal/delivery/export_test.go

@@ -33,6 +33,11 @@ const (
 	// response is written against this number, so a test has to
 	// be able to name it.
 	ExportMaxBodyLog = maxBodyLog
+
+	// ExportPendingSweepMinAge is how long a delivery must sit at
+	// pending before the sweep treats it as stranded. A test has to
+	// name it to age a row past the bound.
+	ExportPendingSweepMinAge = pendingSweepMinAge
 )
 
 // ExportIsBlockedIP exposes isBlockedIP for testing.
@@ -286,6 +291,26 @@ func (e *Engine) ExportWedgeWorker(release <-chan struct{}) {
 	})
 }
 
+// ExportInflightHeld reports how many deliveries the engine currently
+// owns, so a test can prove ownership is released rather than leaked.
+func (e *Engine) ExportInflightHeld() int {
+	return e.inflight.held()
+}
+
+// ExportRetainDelivery takes the first reference on a delivery, as the
+// queueing side does. It lets a test put a delivery into the state a
+// worker or a full channel would, without running the pool.
+func (e *Engine) ExportRetainDelivery(deliveryID string) bool {
+	return e.inflight.retainIdle(deliveryID)
+}
+
+// ExportRecoverRetryingDeliveries exposes recoverRetryingDeliveries.
+func (e *Engine) ExportRecoverRetryingDeliveries(
+	webhookDB *gorm.DB, webhookID string,
+) {
+	e.recoverRetryingDeliveries(webhookDB, webhookID)
+}
+
 // ExportDeliveryCh returns the delivery channel.
 func (e *Engine) ExportDeliveryCh() chan Task {
 	return e.deliveryCh