feat: provider plugin lifecycle management (init/health_check/shutdown)

[luceinaltis]

Context

qracer/provider_catalog.py scans entry points for data/LLM providers, but full lifecycle management is missing. docs/architecture.md lines 80-122 describe a plugin system with providers.toml config-driven registration, but current code uses hardcoded _build_registries().

Goal

Complete the plugin lifecycle so third-party providers can be discovered, configured, health-checked, and shut down cleanly.

Scope

• Add initialize(), health_check(), shutdown() protocol methods to provider base classes
• Wire providers.toml config to provider_catalog.py for declarative provider registration
• Call lifecycle hooks in _build_registries() and Server.shutdown()
• Graceful degradation: if a provider fails health_check(), exclude it with a warning instead of crashing
• Add example third-party provider entry point in docs

Related

• qracer/provider_catalog.py (entry point scanning exists)
• qracer/config/schema/providers.toml (config schema exists)