[Observability 6/10] Integrate Langfuse for MCP tool usage tracking

[concaf]

Context

Add Langfuse integration to track MCP tool usage from the product/analytics perspective. This enables use cases D1-D8 (which tools are used, by whom, how often, usage patterns across projects).

Langfuse is already used in other team projects (e.g., dataverse-mcp-server) for MCP tool tracking.

Scope (to be detailed)

• Evaluate Langfuse Go SDK availability (or REST API integration)
• Track tool calls with: tool name, user identity, parameters, duration, success/failure
• Track session patterns (how LLM agents chain tool calls)
• Capture semantic search relevance scores from get_chunks
• Hash usernames for privacy (same pattern as metrics)

Use cases enabled

D1-D8 (see observability plan)

Depends on

• Phase 1 issues ([Observability 1/10] Add OTEL metrics foundation — telemetry package + main.go wiring #308-[Observability 5/10] Add OTEL config to Kubernetes manifests #312)