feat: propagate request_id through pgSettings and metering context

[pyramation]

Summary

Propagates the per-request request_id (already generated by request-logger.ts middleware via X-Request-Id header or crypto.randomUUID()) into PostgreSQL session settings and the LLM metering pipeline.

Changes:

• graphile.ts: adds request.id to pgSettings for both authenticated and anonymous GraphQL requests
• llm-api.ts: adds request.id to getPgSettings() for REST LLM endpoints
• metering.ts: adds requestId to MeteringContext interface; includes request_id in record_usage() billing metadata for both embed and chat operations
• metering-plugin.ts: extracts request.id from pgSettings into the metering context

After this change, request.id is available in SQL via current_setting('request.id', true) — enabling distributed tracing and correlation across Express → GraphQL → billing → inference log layers.

Companion PR: constructive-io/constructive-db#1276 (adds request_id uuid column to inference_log_module)

Review & Testing Checklist for Human

• Verify request.id appears in pgSettings during a real GraphQL request (check via current_setting('request.id', true) in a PG function or trigger)
• Confirm billing record_usage metadata includes request_id — check the billing ledger jsonb after an LLM call

Notes

No new dependencies. The request_id generation already existed in request-logger.ts — this PR just wires it through to pgSettings and metering. The request.id pgSetting is the standard Constructive convention (matching jwt.claims.* pattern).

Link to Devin session: https://app.devin.ai/sessions/2b5a29d83d3f478e8d3d972653b4879c
Requested by: @pyramation

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