feat: add OpenRouter provider for credit/usage monitoring

[omnissiah-comelse]

Summary

Adds OpenRouter as a new provider for tracking API credit usage, spending rates, and rate limits.

What it does

Polls GET /api/v1/auth/key to track:

• Total usage (USD cumulative)
• Daily/weekly/monthly usage breakdowns
• Credit limit and remaining balance (when a spending limit is configured)
• Rate limit info (requests per interval)
• Free tier detection

Why

OpenRouter is widely used by developers running multi-model setups (Claude, GPT, Llama, Mistral, etc.) through a single API. There's currently no way to monitor OpenRouter quota/spending in onWatch — this fills that gap.

Implementation

Follows the existing provider pattern (Z.ai / MiniMax):

File	Purpose
internal/api/openrouter_client.go	HTTP client with Bearer auth
internal/api/openrouter_types.go	Response structs, snapshot type
internal/store/openrouter_store.go	SQLite storage + cycle tracking
internal/tracker/openrouter_tracker.go	Delta tracking, rate projections
internal/agent/openrouter_agent.go	Polling agent

Integration points modified:

• internal/config/config.go — OPENROUTER_API_KEY env var + HasProvider()
• internal/store/store.go — Schema tables + indexes
• internal/web/handlers.go — Dashboard endpoints (current, history, cycles, insights, sessions)
• main.go — Client/tracker/agent wiring + banner display
• .env.example — New env var entry

Testing

• go build ./... — clean
• go vet ./... — clean
• Tested locally with a real OpenRouter API key — polling works, snapshots stored, cycle tracking functional

Dashboard

The OpenRouter tab shows:

• Credits usage card (usage vs limit when configured, or total usage)
• Daily/weekly/monthly usage breakdowns
• Usage rate projections
• Historical chart data
• Session tracking
• Cycle overview

Configuration

OPENROUTER_API_KEY=sk-or-v1-your-key-here

No auto-detection — requires explicit API key in .env or environment.

[prakersh]

Backend implementation is solid - follows existing provider patterns, parameterized SQL, proper API key redaction, bounded queries. Frontend needs OpenRouter-specific branches in dashboard.html and app.js - will fix in a follow-up.

[prakersh]

Thanks for this contribution @omnissiah-comelse! The backend implementation was solid - clean provider pattern, parameterized SQL, proper key redaction. We've added the frontend integration in #48 (Credits quota card, usage charts, cycle overview, and menubar support).

If onWatch has been useful to you, we'd appreciate a star on the repo - it helps others discover the project!