feat(multiplayer): add server process/assignment-ready registration API

Add POST /v1/servers/{id}/register (and its /api/v1 contract alias),
authenticated by the same workload binding as the result route. A
game server reports its protocol version and image digest and asks
to advance ALLOCATING -> PROCESS_READY -> ASSIGNMENT_READY; the store
boundary (AdvanceServerRegistration) does this as one idempotent
SERIALIZABLE transaction that also advances every participant's queue
ticket, and gates the final transition on every participant having a
live, unexpired assignment.

Adversarial review of the surrounding routing turned up a pre-existing
bug: contractServerMutation rejected any path containing '/', so the
already-documented /api/v1/servers/{id}/result route (and this new
/register route) 404'd for every real caller despite being declared
in the OpenAPI contract. Fix it to delegate shape validation to
serverMutation, matching how contractQueueMutation handles its own
two-segment paths, and add a regression test covering both contract
routes end to end.

server/cmd/control-plane/main.go

@@ -85,6 +85,7 @@ func newAPIHandler(db *sql.DB, indexes ...api.CandidateIndex) http.Handler {
 		QueueBackend:     store.PostgresQueue{DB: db},
 		ProposalBackend:  api.ProposalProviderFromStore(db),
 		ProposalPromoter: api.ProposalPromoterFromStore(db),
+		ServerRegistrar:  api.ServerRegistrarFromStore(db),
 		Assignment:       api.AssignmentProviderFromStore(db),
 		CandidateIndex:   candidateIndex,
 		ProbeRecorder:    store.PostgresQueue{DB: db},