fix(prisma): support legacy token migration replay

[Vann-Dev]

Summary

• Make historical token migrations replay on non-empty pre-token AgentBridge databases by adding nullable columns, deterministic placeholder backfills, then restoring NOT NULL constraints.
• Document post-upgrade operator steps for legacy databases: review generated AgentIds, rotate the company bearer token, and rerun Agent API smoke checks.

Verification

• Reproduced existing failure on disposable Neon schema seeded with legacy User/Company/Agent/Project/Task rows: 20260508210351_add_bearer_token failed with Null constraint failed: (bearerTokenHash).
• After the fix, replayed the full migration chain against a new disposable non-empty legacy schema: PASS.
• corepack pnpm lint
• DATABASE_URL=postgresql://user:pass@localhost:5432/agentbridge?schema=public AUTH_SECRET=placeholder corepack pnpm typecheck
• DATABASE_URL=postgresql://user:pass@localhost:5432/agentbridge?schema=public AUTH_SECRET=placeholder corepack pnpm build

Notes: build still emits existing Next.js middleware deprecation and jose Edge Runtime warnings.