feat(multiplayer): implement WorkloadVerify without a Kubernetes trust boundary

WorkloadVerify (api.Service.WorkloadVerify) was permanently unwired: both
/v1/servers/{id}/register and /v1/servers/{id}/result always 503, because
the only design considered so far was verifying a Kubernetes-projected
service-account JWT (server/workload/jwt.go), which needs a live cluster's
TokenReview/JWKS endpoint to validate against safely -- something this
sandbox cannot do without guessing at a trust boundary.

The API layer doesn't actually require that specific mechanism. serverMutation
only compares WorkloadBinding.ServerID and .MatchID (server/api/service.go);
AdvanceServerRegistration only uses .MatchID/.ServerID/.AllocationID. Nothing
downstream needs Namespace/ServiceAcct/PodUID/GameServerUID populated.

This adds a self-contained alternative: a short-lived, HMAC-signed token the
control plane mints and verifies with a secret only it holds (server/workload/
signed_token.go), the same trust model domain.SessionStore already uses for
player sessions elsewhere in this codebase. It needs no cluster to verify --
signature + expiry is fully self-contained and unit-testable.

The design's soundness rests on the delivery channel, not the crypto: the
token is meant to reach the allocated GameServer via the same Agones
GameServerAllocation annotation channel allocation.go already uses for
match-id/allocation-id, readable only by that pod's own local SDK sidecar. A
caller presenting this token has already proven, via that channel, that it is
the pod Agones allocated. (Wiring the actual annotation delivery -- extending
agones.Client.Allocate and the supervisor's token source -- is a separate,
follow-up change; this commit lands the verification core it depends on.)

store.AllocationBindingStillValid adds defense-in-depth on top of signature
and expiry: it cross-checks the token's claims against the durable
allocations table (append-only, never leaves 'ALLOCATED'), so a validly-signed
token naming an allocation that was never recorded -- or a real allocation id
paired with a mismatched match/server -- is still rejected.

api.WorkloadVerifierFromSignedToken wires the two together and is now plugged
into cmd/control-plane (new --workload-secret / COSMIC_CLASH_WORKLOAD_SECRET
flag; a startup warning is logged if it's left unset, since the route then
stays 503 exactly as before) and cmd/testkit-api (fixed test secret, since
that binary is test-only already).

Verified: new unit tests in server/workload (signature tamper, wrong secret,
expiry boundary, malformed input) and a new Postgres integration suite in
server/api (real allocation row, real signed token, acceptance / unknown-
allocation rejection / mismatched-triple rejection / the previously-503
Service.WorkloadVerify field itself) -- both run clean with -race across
multiple passes against a live postgres:17-alpine container. Full
`go build ./... && go vet ./... && gofmt -l . && go test ./... -race` and
`go test -tags integration ./... -race` both clean.

server/cmd/testkit-api/main.go

@@ -36,6 +36,7 @@ func main() {
 	listen := flag.String("listen", "127.0.0.1:0", "HTTP listen address; port 0 picks a free port, printed on startup")
 	dsn := flag.String("dsn", os.Getenv("COSMIC_CLASH_POSTGRES_DSN"), "PostgreSQL connection string")
 	migrationDir := flag.String("migrations", "migrations", "directory containing numbered SQL migrations")
+	workloadSecret := flag.String("workload-secret", envOrDefault("COSMIC_CLASH_WORKLOAD_SECRET", "testkit-workload-secret"), "HMAC secret for signed workload tokens; defaults to a fixed test value since this binary is test-only")
 	flag.Parse()
 	if *dsn == "" {
 		fatalf("--dsn or COSMIC_CLASH_POSTGRES_DSN is required")
@@ -65,6 +66,7 @@ func main() {
 		RankedProfileProvider: store.PostgresRankedProfiles{DB: db},
 		Assignment:            api.AssignmentProviderFromStore(db),
 		ProbeRecorder:         store.PostgresQueue{DB: db},
+		WorkloadVerify:        api.WorkloadVerifierFromSignedToken([]byte(*workloadSecret), db),
 		Now:                   func() time.Time { return time.Now().UTC() },
 	}).Handler()
 	listener, err := net.Listen("tcp", *listen)
@@ -107,6 +109,13 @@ func (f fakeSteamLogin) Authenticate(ctx context.Context, ticket string, _ time.
 	return domain.VerifiedIdentity{PlayerID: playerID, SteamID: steamID}, nil
 }
 
+func envOrDefault(name, fallback string) string {
+	if value := os.Getenv(name); value != "" {
+		return value
+	}
+	return fallback
+}
+
 func fatalf(format string, args ...any) {
 	fmt.Fprintf(os.Stderr, "testkit-api: "+format+"\n", args...)
 	os.Exit(1)