Lock DATA_DIR against a second instance (closes #201)

Nothing stopped two processes opening the same DATA_DIR. Both open the
same per-webhook databases, both run delivery recovery over the same
rows, and both deliver: every pending delivery reaches the destination
twice, from nothing worse than an overlapping deploy.

The entry point now takes an exclusive advisory flock(2) on
{DATA_DIR}/webhooker.lock before anything opens a database, and holds it
for the process lifetime. A second process pointed at the same directory
prints a message naming that directory and exits 1. The lock is the
kernel's, not the file's, so a process killed with SIGKILL leaves a lock
file that blocks nothing -- which is what a pidfile would get wrong. The
file is never unlinked: doing so would let the next process lock a fresh
inode while a third still held the old one.

Acquisition lives in internal/datadir rather than in the server's fx
graph, so any entry point touching DATA_DIR takes it the same way, and
ErrLocked lets a caller tell a live deployment from any other failure.
config.DataDir() resolves DATA_DIR once, for both the lock and Config,
so the two cannot disagree.

Regression coverage: a real second process is refused, and a restart
after kill -9 succeeds with the stale lock file in place.

github.com/gofrs/flock carries the lock; its own module minimums pull
testify to v1.11.1 and golang.org/x/sys to v0.37.0.

go.mod

@@ -8,13 +8,14 @@ require (
 	github.com/go-chi/chi v1.5.5
 	github.com/go-chi/cors v1.2.1
 	github.com/go-chi/httprate v0.15.0
+	github.com/gofrs/flock v0.13.0
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/sessions v1.4.0
 	github.com/joho/godotenv v1.5.1
 	github.com/prometheus/client_golang v1.18.0
 	github.com/slok/go-http-metrics v0.11.0
-	github.com/stretchr/testify v1.8.4
+	github.com/stretchr/testify v1.11.1
 	go.uber.org/fx v1.20.1
 	golang.org/x/crypto v0.38.0
 	gopkg.in/yaml.v3 v3.0.1
@@ -49,7 +50,7 @@ require (
 	go.uber.org/zap v1.23.0 // indirect
 	golang.org/x/mod v0.17.0 // indirect
 	golang.org/x/sync v0.14.0 // indirect
-	golang.org/x/sys v0.33.0 // indirect
+	golang.org/x/sys v0.37.0 // indirect
 	golang.org/x/text v0.25.0 // indirect
 	golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d // indirect
 	google.golang.org/protobuf v1.31.0 // indirect