fix: make Vercel FastAPI runtime writable

[CuriosityQuantified]

Problem

The deployed Vercel FastAPI function crashes during startup because it creates media directories and opens SQLite under the read-only deployment filesystem. The legacy catch-all also forwards /api/* without stripping the prefix.

Change

• Route runtime-generated media and the SQLite fallback to writable serverless temp storage
• Support explicit runtime/database path overrides
• Normalize Vercel /api/* requests
• Ensure media mounts precede the UI catch-all
• Use the supported Python 3.12 project configuration
• Add Vercel runtime regression coverage

Verification

• Python 3.12 targeted tests: 6 passed
• Python 3.12 CI-equivalent unit suite: 237 passed, 1 skipped, 6 deselected
• Read-only deployment-copy smoke test: root 200 and /api/simulations 200
• Media serving smoke test: 200
• npm run build: passed

Note

Vercel CLI deployment could not be run locally because the configured Vercel token is invalid; merging this PR will allow the connected Git integration to deploy normally.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
save-the-world	Ready	Preview	Aug 10, 2026 1:48am
sim-local	Ready	Preview	Aug 10, 2026 1:48am