Add zero-infra deployment recipes for Fly.io and Railway

Satya_AI is intentionally designed for **zero-infra** usage (flat files by default, SQLite optional), and deployment examples already cover some lightweight paths. However, many users prefer familiar PaaS platforms like Fly.io and Railway, and would benefit from copy-paste deployment recipes.
Task
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Add deployment recipes for:

1. **Fly.io**
- Minimal Dockerfile / fly.toml
- Guidance on mounting storage and configuring Satya env vars

2. **Railway**
- Minimal config for running the Satya app
- Instructions to configure environment variables and storage

Docs should clearly explain:

- expected monthly cost range for a small Satya instance
- how to set required secrets (tokens, audit keys, etc.)

Implementation Steps
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- [ ] Create `docs/deploy/fly_io.md` with step-by-step instructions
- [ ] Create `docs/deploy/railway.md` with step-by-step instructions
- [ ] Add example config files under `deploy/` or similar (Fly, Railway)
- [ ] Update the main `README.md` with a "Deployment options" section linking to these docs

Resources
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- • Existing deployment examples in the repo (if any)
- • Fly.io and Railway documentation (external)

Why This Matters
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Simple, documented deployment recipes lower friction for non-DevOps users and enable "I deployed Satya on X in 10 minutes" style posts that bring new users and contributors.