fix(api): fix addon invocation completion and rate limit UX

[ericfitz]

Summary

Fixes addon invocation issues where users get 429 errors because invocations never complete:

• Auto-complete invocations by default - Webhooks no longer need to call back unless they opt-in with X-TMI-Callback: async header
• Increase default max active invocations from 1 to 3 concurrent
• Enhanced 429 response with blocking invocation details (IDs, expiry times, retry-after)
• Auto-infer server base URL for callback URLs if not explicitly configured

Changes

File	Change
internal/config/config.go	Add BaseURL field with GetBaseURL() auto-inference
api/addon_invocation_worker.go	Use configured base URL, implement X-TMI-Callback auto-complete
api/addon_invocation_quota_store.go	Change DefaultMaxActiveInvocations from 1 to 3
api/addon_invocation_store.go	Add ListActiveForUser interface and Redis implementation
api/addon_rate_limiter.go	Enhanced 429 response with blocking_invocations array
api/request_utils.go	Add Retry-After header for 429 responses
cmd/server/main.go	Wire up base URL to addon invocation worker
docs/developer/addons/addon-development-guide.md	Document callback modes

Behavior Changes

1. Invocations auto-complete by default - When webhook returns 2xx without X-TMI-Callback: async, invocation is marked completed
2. 3 concurrent invocations allowed (was 1)
3. 429 responses include helpful details:
   • blocking_invocations[] with invocation IDs, addon IDs, status, expiry times
   • retry_after seconds until oldest invocation times out
   • Retry-After HTTP header per RFC 6585
4. Base URL auto-inferred from interface, port, and tls_enabled if not set

Test plan

• make lint - 0 issues
• make build-server - SUCCESS
• make test-unit - All passed
• Manual test: invoke addon, verify auto-complete
• Manual test: invoke 3+ addons, verify 429 with details

🤖 Generated with Claude Code