feat: add per-user model cost dashboard

[QueryPlanner]

What

Add provider-aware model cost tracking and dashboard visibility by user, session, and trace.

Why

OpenRouter BYOK responses can still expose usage cost, but Blacki previously retained token counts without monetary cost fields. This adds exact/provider and clearly labeled estimated cost reporting without storing prompts, responses, or tool arguments.

How

• Capture usage.cost, upstream inference cost, LiteLLM estimates, and token usage at the LiteLLM boundary.
• Persist identity-linked, idempotent records in a content-free SQLite ledger.
• Add cumulative, UTC-month, average-user-monthly, and cost-coverage metrics.
• Expose costs in overview, users, sessions, session detail, and traces.
• Document BLACKI_COST_LEDGER_PATH and distinguish unavailable historical records from zero.
• Use official OpenRouter references: https://openrouter.ai/docs/cookbook/administration/usage-accounting and https://openrouter.ai/docs/guides/overview/auth/byok

Tests

• uv run ruff format
• uv run ruff check
• uv run mypy .
• uv run pytest --cov=src --cov-report=term-missing (1,633 passed, 1 skipped, 100.00% coverage)
• npm run build
• node --check src/blacki/dashboard/static/dashboard.js

Related Issues

• No issue was supplied for this feature.