Add full demo seed mode, admin API, and dashboard demo button

[Spbd1]

Motivation

• Provide a turnkey local demo mode so developers can spin up a complete experiment dataset for manual testing and UI previews without manual data entry.
• Ensure admin workflows and dashboards display meaningful charts immediately after seeding by generating synthetic decisions and resolving early rounds through the engine.

Description

• Added a deterministic demo seeding implementation at packages/db/src/demoSeed.ts that creates an admin user, four treatment demo servers, 10x10 maps, 20 anonymous participants per server, parcel assignments, an active 7-round season, synthetic decisions for rounds 1–3 (production, investments, safe assets, public contributions, contracts, lobbying, exits), resolves rounds 1–3 via the engine, and records contracts/events/treasury/player round states.
• Exposed a runnable script and command: packages/db/scripts/seed-demo.ts and a new workspace script pnpm seed:demo that runs the demo seed (pnpm --filter @parcel-society/db seed:demo).
• Added a development-only admin API endpoint POST /api/admin/demo at apps/web/app/api/admin/demo/route.ts and a dashboard action button labeled Create Demo Experiment (visible only when NODE_ENV === "development") in apps/web/app/admin/page.tsx which calls the endpoint and records an audit action.
• Updated admin auth defaults and UI for local development and docs: default demo credentials set to ADMIN_EMAIL=admin@example.com and ADMIN_PASSWORD=changeme (.env.example, apps/web/lib/api/auth.ts, and admin login form), and README documentation describing pnpm seed:demo and demo login details.

Testing

• Ran Prettier formatting checks with pnpm exec prettier --check ... which passed for the modified files.
• Ran git diff --check to ensure no whitespace/patch issues; the check passed.
• pnpm install / typecheck / lint / full pnpm seed:demo were attempted but blocked in this environment due to missing node_modules (registry 403 for @types/node) and missing runtime tsx, so full runtime/typecheck verification could not complete here.
• Notes: the demo seed is deterministic and designed to be re-runnable (it replaces existing demo servers), but please run pnpm install, pnpm --filter @parcel-society/db db:generate, pnpm --filter @parcel-society/db db:migrate, then pnpm seed:demo locally to execute and verify end-to-end behavior.

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Codex Task