feat: add cron jobs + internal tools use cases, local dev guide

[YairEtzion]

Summary

• Cron Jobs use case page — "Your cron job has a plaintext API key. So does the attacker." Pain points: keys in plaintext, can't distinguish cron from attacker, rotation across servers. Code examples for billing-sync script, API server verification, and CLI setup.
• Internal Tools use case page — "Five developers share one admin key. Your SOC2 auditor is not impressed." Pain points: no individual accountability, offboarding nightmare, SOC2 compliance gap. Code examples for admin scripts with audit logging and per-developer revocation.
• Site navigation updated — both pages registered in use cases index (2x2 grid), nav dropdown, and footer.
• Local Development section added to integration guide (markdown + Svelte site) — setup with --backend encrypted-file --passphrase "dev", same code in dev and production, AUTH_MESH_PASSPHRASE added to env vars table.

Test plan

• bun run build — all pages prerender (verified: cron-jobs.html, internal-tools.html in build output)
• Visual check: /use-cases/cron-jobs and /use-cases/internal-tools render correctly
• Visual check: /use-cases index shows 4 cards in 2x2 grid
• Visual check: nav dropdown shows 4 entries
• Visual check: /docs/integration has Local Development section before Environment Variables