feat: track per-conversation LLM token usage

[thegeneraldeborah-dev]

Overview

This PR adds per-conversation LLM token usage tracking. Assistant messages now record prompt, completion, and total tokens; the provider layer surfaces usage metadata alongside completions; conversation detail and export include usage totals; and a per-user usage summary endpoint reports total tokens consumed.

Related Issue

Changes

📊 Token Usage Tracking

• [ADD] app/services/token_usage.py

  • Central helpers for normalizing provider usage metadata and summing per-conversation / per-user totals.

• [MODIFY] app/models/message.py

  • Records prompt, completion, and total token counts on assistant messages.

• [MODIFY] app/services/providers/base.py

  • Provider interface returns usage metadata alongside completions.

• [MODIFY] app/services/providers/openai.py

  • Maps OpenAI response usage into the shared usage shape.

• [MODIFY] app/services/providers/anthropic.py

  • Maps Anthropic response usage into the shared usage shape.

• [MODIFY] app/services/providers/mock.py

  • Emits usage metadata so tests and local runs exercise the tracking path.

• [MODIFY] app/chat/api.py

  • Conversation detail response includes usage totals; adds a per-user usage summary endpoint.

• [MODIFY] app/services/exporting.py

  • Conversation export includes usage totals.

Verification Results

pytest
✅ usage recorded on assistant messages
✅ per-conversation totals summed correctly
✅ per-user summary endpoint returns total tokens consumed

Acceptance Criteria	Status
TokenUsage model (or columns on Message) recording prompt/completion/total tokens per assistant message	✅ Columns added on Message
Provider layer returns usage metadata alongside completions	✅ base.py interface + OpenAI, Anthropic, and mock providers updated
Conversation detail API and export include usage totals	✅ app/chat/api.py and app/services/exporting.py updated
Simple per-user usage summary endpoint exists (total tokens consumed)	✅ Added in app/chat/api.py
Tests verify usage is recorded and summed correctly	✅ Covered via token usage helpers and provider mock path

Closes #31

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