Feature/license headers and fixes

[Saboleee]

Summary

This PR addresses four issues: SPDX license headers, JSON-RPC error test coverage, startup availability signaling, and release documentation synchronization.

Changes

#116 — Add SPDX license headers to source files

• Added SPDX-License-Identifier: Apache-2.0 to all Rust source files in api/src/
• Ensures compliance with per-file license header requirements

#109 — Add tests for JSON-RPC error paths

• Extracted RPC response processing into process_rpc_envelope() function
• Added unit tests for envelope-level error (resp.error)
• Added unit tests for simulation-level error (result.error)
• Both error branches now map to IndexError::Rpc as designed

#115 — Return 503 with Retry-After until first refresh succeeds

• Added initialization tracking to AppState via atomic flag
• Snapshot marked initialized only after first successful refresh
• Clients requesting data before initialization receive 503 Service Unavailable with Retry-After header
• Allows clients to distinguish "no data yet" from "genuinely empty"

#110 — Add release checklist to sync CHANGELOG and docs

• Added "Release checklist" section to CONTRIBUTING.md
• Documents steps to ensure API/contract changes sync between CHANGELOG and docs
• Provides maintainers with a clear workflow for version releases

Closes #109, Closes #110, Closes #115, Closes #116

[drips-wave]

@Saboleee Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits