feat: migrate observability to Micrometer

[me0106]

Summary

Closes #124

• Replace the old OpenTelemetry tracing/decorator path with Micrometer Observation-based instrumentation across extraction, retrieval, rerank, graph, tier, and strategy components.
• Add Micrometer observability plugin/starter wiring for observation handlers, business metrics recording, and Spring Boot auto-configuration.
• Rework retrieval debug trace collection to use request-scoped recorders fed by observation contexts, with bounded API trace output and inline documentation.
• Update memind-server wiring/configuration for Micrometer observability and OpenTelemetry export through Spring Boot integration.

Review Corrections

This update addresses the follow-up observability review findings:

• Removed duplicate Spring Boot ObservationHandler registration; handler beans are now discovered and registered by Spring Boot.
• Aligned timeout fallback status across the API result, retrieval trace, and metrics as DEGRADED.
• Created a fresh mutable Observation.Context for every Insight generator subscription.
• Disabled Memind observation creation when memind.server.observability.enabled=false.
• Corrected the result-merge deduped_count metric.
• Added observation coverage for custom Reranker implementations without double-observing LlmReranker.
• Marked query-expansion and rerank fallback paths as degraded.
• Allowed matching GlobalObservationConvention beans to override component defaults.
• Removed the unconditional Spring Boot debug: true setting.

Test Plan

• mvnd test
• 36 reactor modules succeeded
• 0 test failures