Add a webhooker resetpw subcommand and a bootstrap banner (closes #208) (#239)

The admin bootstrap password was printed once, as one line among roughly
45 fx lines, and under docker run -d went to container logs subject to
rotation. There was no reset path at all -- no subcommand, no forgot-password
flow, no env override -- so recovery meant hand-deleting the users row from
webhooker.db, which was documented nowhere.

Adds webhooker resetpw [-generate] <username>. The password is read from
stdin or generated with the existing crypto/rand helper, never taken from
argv where /proc would publish it. It reuses the existing Argon2id hashing
rather than reimplementing the parameters, and writes a single UPDATE only
after the hash is complete, so no failure can leave an account with no
usable password. An unknown username is a hard error and never creates an
account.

It refuses to run against a DATA_DIR held by a live instance, via the
exclusive lock from #201. DATA_DIR and webhooker.db are checked to exist
before the lock is acquired, so a mistyped path creates nothing -- neither
a directory tree nor a stray lock file.

The bootstrap password now appears exactly once, in a distinct banner
written straight to a caller-named writer rather than as an fx log line.

internal/database/export_test.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"context"
+	"io"
 	"log/slog"
 	"os"
 	"time"
@@ -66,6 +67,13 @@ func (r *RetentionReaper) ExportSetInterval(d time.Duration) {
 	r.interval = d
 }
 
+// ExportSetBannerOut redirects the first-boot credentials banner, so a
+// test can read what the operator would have seen. It must be called
+// before the fx start hook runs, which is where the account is seeded.
+func (d *Database) ExportSetBannerOut(w io.Writer) {
+	d.bannerOut = w
+}
+
 // DummyPasswordHashForTest exposes the encoded hash that unknown
 // usernames are verified against.
 func DummyPasswordHashForTest() string {