feat: implement graceful draining and outbound call control for SIP

[biswajitpain]

Introduce a graceful shutdown mechanism for the SIP service.

• Add StartDrain methods to sip.Client, sip.Server, and sip.Service. These methods stop accepting new inbound SIP INVITEs and new outbound CreateSIPParticipant requests, respectively, while allowing existing calls to complete.
• Integrate the StartDrain call into the service shutdown sequence in service.Run() to immediately prevent new call establishments.
• Add a DisableOutboundCalls configuration option to prevent new outbound SIP calls. When enabled, CreateSIPParticipant requests will be rejected, and affinity will be zeroed out.
• Enhance the health check endpoint:
• Expose /healthz in addition to /.
• Implement a 30-second grace period after service stop before exiting, allowing load balancers to detect unhealthy status.
• Update .gitignore to include .claude.

Changes:

• .gitignore (2 lines, 7 characters)
• cmd/livekit-sip/main.go (2 lines, 219 characters)
• pkg/config/config.go (4 lines, 274 characters)
• pkg/service/service.go (26 lines, 1142 characters)
• pkg/sip/client.go (6 lines, 213 characters)
• pkg/sip/inbound.go (7 lines, 429 characters)
• pkg/sip/server.go (6 lines, 220 characters)
• pkg/sip/service.go (14 lines, 522 characters)
• test/cloud/service.go (2 lines, 263 characters)

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