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/target_database_archive.go

@@ -1,7 +1,6 @@
 package delivery
 
 import (
-	"database/sql"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -12,6 +11,7 @@ import (
 
 	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 )
 
@@ -30,13 +30,13 @@ const (
 	// path: open the archive file, creating it if missing, so a
 	// first write (or a write after the operator moved the file
 	// away) recreates it.
-	archiveModeCreate = "rwc"
+	archiveModeCreate = database.SQLiteModeCreate
 
 	// archiveModeExisting is the SQLite URI mode used by the idle
 	// sweep: open read-write but never create. A sweep must never
 	// conjure an empty archive file for a webhook that has a
 	// database target but has never received an event.
-	archiveModeExisting = "rw"
+	archiveModeExisting = database.SQLiteModeExisting
 )
 
 var (
@@ -273,9 +273,11 @@ func (w *archiveWriter) open(expiry time.Duration) error {
 func (w *archiveWriter) openMode(
 	mode string, expiry time.Duration,
 ) error {
-	dbURL := fmt.Sprintf("file:%s?mode=%s", w.path, mode)
-
-	sqlDB, err := sql.Open("sqlite", dbURL)
+	// Opened through database.OpenSQLite so an archive file carries
+	// the same WAL journaling, busy timeout, immediate-transaction
+	// locking, and pool bounds as every other database file. See
+	// internal/database/sqlite_open.go.
+	sqlDB, err := database.OpenSQLite(w.path, mode)
 	if err != nil {
 		return fmt.Errorf(
 			"opening archive database %s: %w", w.path, err,