feat: replace request_type with service + operation in inference log inserts

[pyramation]

Summary

Coordinates with constructive-db PR #1281 (merged) which added service and operation fields to inference_log_module replacing the old request_type field.

Changes in graphile-llm/src/metering.ts:

• InferenceLogEntry interface: requestType → service + operation fields
• Added cacheReadTokens, cacheWriteTokens (for Anthropic/OpenAI cache billing)
• Added rawUsage (jsonb for raw provider response audit)
• logInferenceUsage() SQL updated to INSERT all new columns (20 params)
• Embedding calls: service: 'embedding', operation: 'create'
• Chat calls: service: 'llm', operation: 'chat'

Changes in graphql/server/src/middleware/llm-api.ts:

• logInference() function: requestType → service + operation
• SQL INSERT updated to 11 params
• Both callers (streaming + non-streaming) updated

Review & Testing Checklist for Human

• Verify the INSERT column order matches the inference_log_module field order from constructive-db PR #1281
• After merging, do an e2e test: send a chat message → verify usage_log_inferences has service='llm', operation='chat'
• Verify embedding requests produce service='embedding', operation='create'

Notes

The service field uses a fixed vocabulary of ~8 values: llm, embedding, tts, stt, ocr, image_gen, search, compute. The operation field is higher cardinality and service-specific.

cacheReadTokens, cacheWriteTokens, and rawUsage are passed as null for now — they'll be populated when Anthropic/OpenAI provider support is added.

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

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