- generatePkce() using crypto.randomBytes base64url verifier and SHA256 challenge
- generateAuthUrl() builds Google OAuth URL with PKCE params for Gemini scopes
- exchangeCode() POSTs to Google token endpoint with code_verifier
- storeTokens() upserts google_gemini_oauth_token via secretService
- resolveTokens() retrieves and parses stored tokens by companyId