Throwaway guests, shared AI allowance, demo mode

[tim4431]

• Guests keep nothing: every guest login mints a throwaway guest-<random> account with its own workspace, deleted after guest_ttl_hours by the session middleware and a background sweeper (gamma/guests.py, one workspaces.delete_account). Optional GAMMA_GUEST_SEED starter zip, GAMMA_GUEST_MAX cap. Migration step 20 removes the old shared guest account.
• Shared AI allowance: tokens per account per rolling 24 h on the admin's shared provider entries (separate limits for accounts and guests), enforced in ai_client.open_ai; own keys are never metered. 429 with a readable detail on chat, translate, metadata and transcribe.
• Demo mode (demo_mode setting or GAMMA_DEMO=1): the login page leads with Try the demo, the first-paper tour is offered on arrival, guide progress lives in sessionStorage. Settings → Server → Guests holds the lifetime and the switch.
• demo.gammapdf.com: a demo service in cloud/deploy/compose.yml behind the wildcard Caddy site, demo.env.example, and the update-demo-server skill (branch build of docker.yml, pinned by sha-<short>).
• Cloud sync logic update.
• Docs: new docs/dev/guests.md; api, workspaces, user_db, migrations, ai, settings, onboarding and github_actions updated.

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