starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown

[avi-starkware]

Installs a process-wide panic hook that emits one structured event=panic
tracing event with location and backtrace, and bridges SIGTERM/SIGINT into
ServerHandle::stop (extracted to server/shutdown.rs, keeping main.rs to
a single call) so container teardown becomes visible in logs and in-flight
proofs get to finish. A second signal while the drain is still in progress
forces exit(1) so an operator can always reclaim a stuck process
(tokio-rs/tokio#7905); once the server has stopped cleanly, a late signal no
longer flips the exit code.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[cursor]

PR Summary

Low Risk
Changes are limited to process lifecycle, logging, and signal handling; no proving, RPC, or auth logic is modified.

Overview
Adds operational hardening for the transaction prover service: panics and container teardown are now visible in structured logs, and shutdown can drain in-flight work.

A process-wide panic hook (install_panic_hook, after tracing init) replaces default stderr output with a single tracing error! event (event=panic) carrying location, message, and backtrace—without changing abort-on-panic behavior. Payload extraction is covered by a unit test.

SIGTERM/SIGINT are bridged via spawn_signal_bridge to ServerHandle::stop() so the first signal starts a graceful drain; structured shutdown_started / shutdown_complete events are logged, and a second signal during drain forces exit(1) to avoid Tokio swallowing follow-up signals. Dependencies gain tokio signal and tracing-subscriber json for JSON log format support.

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[avi-starkware]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• starknet_transaction_prover: log resolved config at startup (secrets redacted) #14684
• starknet_transaction_prover: document observability surface in README #14203
• starknet_transaction_prover: origin-level breadcrumbs for validation errors #14172
• starknet_transaction_prover: /health returns 503 when service is saturated #14171
• starknet_transaction_prover: bump prover_panics_total on panic hook fire #14170
• starknet_transaction_prover: HTTP request count + latency + in-flight metrics #14169
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167
• starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown #14166 👈 (View in Graphite)
• starknet_transaction_prover,tower_ohttp: OHTTP-unlinkable request-id for decapsulated content #14222
• starknet_transaction_prover: per-request structured log with request-id #14165
• starknet_transaction_prover: unify HTTP middleware stack via macro #14286
• main-v0.14.3

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