feat: Gmail OAuth - Connect with Google for individual org accounts

[benzntech]

Summary

• Adds google_oauth as a new ingestion provider type for individual Gmail organizational accounts
• Introduces GoogleOAuthCredentials type (email, accessToken, refreshToken) alongside the existing google_workspace service-account provider — no breaking changes
• Includes full RapidSpec proposal, tasks, investigation, research, and spec delta in rapidspec/changes/gmail-oauth-connect/

Problem

Connecting a Gmail account currently requires pasting a raw Google Service Account JSON blob + an admin email — a highly technical process that breaks the UX expectations of modern users.

Solution

A new google_oauth provider that lets individual users click "Connect with Google", complete a standard OAuth consent flow, and have their mailbox added automatically. No manual credential fields.

OAuth Flow

User clicks "Connect with Google"
  → GET /api/v1/oauth/google/authorize (authenticated)
  → Redirect to Google consent screen (gmail.readonly + userinfo.email)
  → GET /api/v1/oauth/google/callback
  → Exchange code → store tokens → create IngestionSource
  → Redirect to /dashboard/ingestions

What's in this PR

• packages/types/src/ingestion.types.ts — GoogleOAuthCredentials interface + google_oauth in provider union
• rapidspec/changes/gmail-oauth-connect/ — full proposal spec (implementation continues in follow-up PRs)

Test plan

• pnpm build passes with updated types
• Remaining implementation tasks tracked in rapidspec/changes/gmail-oauth-connect/tasks.md

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