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.

cmd/webhooker/main.go

@@ -17,6 +17,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/healthcheck"
 	"sneak.berlin/go/webhooker/internal/logger"
 	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/resetpw"
 	"sneak.berlin/go/webhooker/internal/server"
 	"sneak.berlin/go/webhooker/internal/session"
 )
@@ -48,6 +49,11 @@ import (
 // and can still consume the whole budget on their own.
 const stopTimeout = 5 * time.Second
 
+// exitUsage is the status for a command line this binary cannot make
+// sense of, kept distinct from the 1 a refusal exits with so that a
+// caller can tell "called wrong" from "declined".
+const exitUsage = 2
+
 // Build-time variables set via -ldflags.
 //
 //nolint:gochecknoglobals // Build-time variables injected by the linker.
@@ -60,7 +66,54 @@ func main() {
 	globals.Appname = appname
 	globals.Version = version
 
-	os.Exit(run(os.Stderr))
+	os.Exit(dispatch(os.Args[1:], os.Stdin, os.Stdout, os.Stderr))
+}
+
+// dispatch routes the command line to a subcommand.
+//
+// No arguments runs the server, which is what the image's CMD and
+// every existing deployment invoke; that path is unchanged, including
+// where the DATA_DIR lock is taken relative to building the fx graph
+// and how fx propagates a non-zero exit itself.
+func dispatch(
+	args []string,
+	stdin io.Reader,
+	stdout, stderr io.Writer,
+) int {
+	if len(args) == 0 {
+		return run(stderr)
+	}
+
+	switch args[0] {
+	case resetpw.Name:
+		return resetpw.Run(args[1:], stdin, stdout, stderr)
+	case "help", "-h", "-help", "--help":
+		usage(stdout)
+
+		return 0
+	default:
+		_, _ = fmt.Fprintf(
+			stderr, "%s: unknown subcommand %q\n", appname, args[0],
+		)
+		usage(stderr)
+
+		return exitUsage
+	}
+}
+
+// usage lists what the binary can be asked to do.
+func usage(w io.Writer) {
+	_, _ = fmt.Fprintf(w, `usage: %s [subcommand]
+
+With no subcommand, runs the webhooker server.
+
+Subcommands:
+  %s [-generate] <username>
+        Set an existing account's password on a stopped deployment.
+        Recovers an admin account whose bootstrap password was lost.
+  help
+        Print this message.
+`, appname, resetpw.Name)
 }
 
 // run takes the exclusive DATA_DIR lock, then runs the application