docs: move deployment to the top and detail the self-hosted Pi + Google auth flow

[tyrellcurry]

Moves the Deployment section up near the top (right after the live link), and expands it to actually showcase what's interesting about it rather than just naming the tools:

• The API and Postgres aren't on a cloud provider at all, they run on a Raspberry Pi 5 at home, reachable over a real domain with Caddy handling TLS.
• Walks through how Google sign-in actually works against that same Postgres instance: exchange the auth code, verify the ID token, upsert a users row keyed by the Google subject ID, issue a session row whose bearer token the frontend holds, every invoice query scoped to that session's owner at the SQL layer. A guest gets the identical session mechanism, just without a user row attached.

Trimmed the "Auth" bullet under What's here to a pointer, so the story isn't told twice.

Built on top of develop as it stands now (your own #70 and #71 already merged), kept the plain link style from #70 rather than reintroducing the centered wrapper.

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Project	Deployment	Actions	Updated (UTC)
invoice-app	Ready	Preview	Aug 8, 2026 3:14am

[Copilot]

Pull request overview

Reworks the root README to highlight the project’s deployment architecture and end-to-end auth/session data flow earlier, making the app’s “real-world” infrastructure/story more visible to readers.

Changes:

• Moves the “Deployment” section near the top of the README (right after the live link).
• Expands “Deployment” with concrete details on the self-hosted Raspberry Pi backend and Google OAuth → Postgres-backed session flow.
• Trims the “Auth” bullet under “What’s here” to avoid duplicating the full explanation.

Suppressed comments (1)

README.md:22

• Comma splice: this reads more cleanly as two sentences.

- **Auth**: Google OAuth alongside an ephemeral guest mode, unified under one session model in Postgres, see [Deployment](#deployment) above for how that actually works.

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