clients: add instant RTDT calls

[miki-totefu]

No description provided.

[vctt94]

I tested an instant audio call between bruig and brclient, and it worked fine.

When setting up the server, if cookies are missing, the server crashes:

vctt@vctt-pc:~/projects/bisonrelay/brserver$ go build && ./brserver 
2025-10-17 10:43:33.347 [INF] SERV: Initialized FileSystem Database backend
2025-10-17 10:43:33.347 [INF] SERV: brserver version: 0.2.3+eafff7e85, RPC Protocol: 10
2025-10-17 10:43:33.355 [INF] SERV: Start of day
2025-10-17 10:43:33.355 [INF] SERV: Our outer fingerprint: aa1318870ef8dd4671318a15859c4375d043bc30030870af3fe9bb19fc46422e
2025-10-17 10:43:33.355 [INF] SERV: Our inner fingerprint: a30c2d536d75e9faf38c01ceaff2423c440ebf8c171e04a475c5058701f1efcc
2025-10-17 10:43:33.355 [INF] SERV: Profiler enabled on http://127.0.0.1:6060/debug/pprof
2025-10-17 10:43:33.362 [INF] SERV: Initialized dcrlnd payment subsystem using node 03919ec803888974d01b2042bd5e4cda430254bf61471b4036a677e4a5d70d382a
2025-10-17 10:43:33.362 [INF] SERV: Push data rate: 1.00000000 DCR/GB
2025-10-17 10:43:33.362 [INF] SERV: Listening on 0.0.0.0:12345
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x677def]

goroutine 85 [running]:
golang.org/x/crypto/salsa20/salsa.HSalsa20(0xc00064b9c0, 0x18?, 0x18?, 0x18?)
	/home/vctt/go/pkg/mod/golang.org/x/[email protected]/salsa20/salsa/hsalsa20.go:18 +0xf
golang.org/x/crypto/nacl/secretbox.setup(0xc00063a258?, 0xc00064ba40, 0xc00063a258, 0xc00064b9e0?)
	/home/vctt/go/pkg/mod/golang.org/x/[email protected]/nacl/secretbox/secretbox.go:52 +0x45
golang.org/x/crypto/nacl/secretbox.Seal({0xc00063a270, 0x18, 0x18}, {0xc00064bad8, 0x28, 0x28}, 0x0?, 0x0?)
	/home/vctt/go/pkg/mod/golang.org/x/[email protected]/nacl/secretbox/secretbox.go:79 +0x7a
github.com/companyzero/bisonrelay/rpc.(*RTDTSessionCookie).Encrypt(0xc000610360?, {0x0, 0x0, 0x0}, 0x0)
	/home/vctt/projects/bisonrelay/rpc/rtdt.go:107 +0x13b
github.com/companyzero/bisonrelay/server.(*ZKS).handleCreateRTDTSession(0xc0001048c0, {0x153e7e0?, 0xc00062c050?}, 0xc00063e000, {{0xc0005de6a0?, 0xc00062c050?}, 0xc00063e000?, 0x20?, 0xc0?}, {0x2})
	/home/vctt/projects/bisonrelay/server/rtdt.go:84 +0x473
github.com/companyzero/bisonrelay/server.(*ZKS).sessionReader(0xc0001048c0, {0x153e7e0, 0xc00062c050}, 0xc00063e000)
	/home/vctt/projects/bisonrelay/server/session.go:438 +0xe4d
github.com/companyzero/bisonrelay/server.(*ZKS).runNewSession.func3()
	/home/vctt/projects/bisonrelay/server/session.go:513 +0x25
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/home/vctt/go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:78 +0x50
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 82
	/home/vctt/go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x96

Adding a nil check for z.rtCookieKey in handleCreateRTDTSession would prevent this crash.