Add workspace-scoped mail support

[swhan0329]

/claim #1

Summary

• create or repair each user's personal workspace at sign-in and from the workspace API
• add workspace create/list APIs plus teammate invite and accept routes backed by the existing Organization, Membership, and EmailAccount models
• wire the mail UI to real workspaces, select workspace email accounts, and pass workspace/account scope into the Gmail thread API
• validate workspace membership before returning scoped threads and keep newly created workspaces usable by adding the creator's email account immediately

Notes

This is a focused slice for the Slack-style workspace flow in #1. It uses the existing Prisma workspace models rather than introducing new tables.

Verification

• pnpm --filter inbox-zero-ai exec tsc --noEmit
• pnpm --filter @inboxzero/resend exec tsc --noEmit
• env NODE_ENV=test POSTGRES_PRISMA_URL=postgresql://user:pass@localhost:5432/db POSTGRES_URL_NON_POOLING=postgresql://user:pass@localhost:5432/db NEXTAUTH_URL=http://localhost:3000 NEXTAUTH_SECRET=secret GOOGLE_CLIENT_ID=dummy GOOGLE_CLIENT_SECRET=dummy OPENAI_API_KEY=dummy UPSTASH_REDIS_URL=https://example.com UPSTASH_REDIS_TOKEN=dummy GOOGLE_PUBSUB_TOPIC_NAME=dummy LEMON_SQUEEZY_API_SECRET=dummy LEMON_SQUEEZY_SIGNING_SECRET=dummy TINYBIRD_TOKEN=dummy LANGBASE_URL=https://example.com LANGBASE_SUMMARIES_SECRET=dummy LANGBASE_CATEGORIES_SECRET=dummy LANGBASE_ACTIONS_SECRET=dummy LANGBASE_URGENT_SECRET=dummy NEXT_PUBLIC_PRO_PAYMENT_LINK=https://example.com/pro NEXT_PUBLIC_LIFETIME_PAYMENT_LINK=https://example.com/lifetime NEXT_PUBLIC_LIFETIME_PLAN_ID=dummy pnpm --filter inbox-zero-ai lint
• git diff --check

Lint passes with existing warnings in unrelated files: CommandK, mail-list, and single-image-dropzone.

[swhan0329]

@algora-pbc /claim #1