feat: add reports page with cost/token charts and CSV export

[diogenesc]

New /reports page in the dashboard for historical cost and usage analysis.

What's implemented

Backend — new app/modules/reports/ module:

• GET /api/reports endpoint with filters (start_date, end_date, account_id, model)
• SQL aggregations by day, model, and account on the request_logs table
• SQLite and PostgreSQL support

Frontend — new /reports page:

• Summary cards (total cost, tokens, requests)
• Area chart: Cost per Day
• Area chart: Tokens per Day (input/output)
• Donut: Cost Distribution by Model
• Daily detail table with CSV export
• Filters with presets 7d/30d/90d and custom date picker
• Consistent design with shadcn/ui + Tailwind + Recharts

[Soju06]

Hermes owner-review blocker pass (current head c73f1b9):

• SQLite /api/reports path currently crashes. app/modules/reports/repository.py:54 calls cast(RequestLog.requested_at, String).substring(0, 10), but SQLAlchemy Cast has no .substring() comparator. Local smoke result: AttributeError: Neither 'Cast' object nor 'Comparator' object has an attribute 'substring'. Use the existing SQLite func.strftime(...)-style date aggregation pattern instead.
• OpenSpec merge gate is missing for this behavior/API/UI change. The PR adds GET /api/reports and a new dashboard /reports page, but git diff --name-only origin/main...HEAD -- openspec/changes openspec/specs openspec is empty. Please add a real openspec/changes/<slug>/ change with parseable deltas for the new reports API/UI contract.
• Current backend lint fails. uvx ruff check app/modules/reports app/dependencies.py app/main.py reports I001 import ordering in app/modules/reports/service.py:1.

Also worth fixing before re-review: nullable request_logs.account_id can flow into AccountCostEntry.account_id: str (repository.py:175-178, schemas.py:25-29), which can break response construction for historical rows whose account was deleted.