Add provider-agnostic OAuth support for Google login via PropelAuth

[Copilot]

What does the PR do?

PropelAuth manages OAuth providers (Slack, Google, etc.) server-side. Backend had hardcoded Slack-specific user ID handling (oauth2|slack|T1Q7936BH-{id}). This prevented adding Google login without code changes.

Changes:

• New OAuth utilities module (common/utils/oauth_providers.py)

  • Provider detection, user ID normalization, Slack ID extraction
  • Handles any PropelAuth OAuth format: oauth2|{provider}|{identifier}
  • 34 unit tests with edge case coverage

• Updated 8 backend files to use provider-agnostic utilities:

  • db/firestore.py, services/users_service.py, common/utils/firebase.py
  • services/volunteers_service.py, api/teams/teams_service.py, api/messages/messages_service.py
  • scripts/add_new_praise_columns.py, services/problem_statements_service.py

• Documentation (docs/OAUTH_SETUP.md)

  • PropelAuth dashboard configuration steps
  • Google Cloud Console OAuth setup

Result: Google (or any OAuth provider) can now be enabled via PropelAuth dashboard without backend changes. Existing Slack login unaffected.

# Before: Hardcoded Slack handling
SLACK_PREFIX = "oauth2|slack|T1Q7936BH-"
slack_user_id = f"{SLACK_PREFIX}{user_id}"

# After: Provider-agnostic
from common.utils.oauth_providers import normalize_slack_user_id, is_oauth_user_id
normalized_id = normalize_slack_user_id(user_id) if not is_oauth_user_id(user_id) else user_id
# Handles: oauth2|slack|..., oauth2|google-oauth2|..., oauth2|github|..., etc.

Type of change

• Breaking Change
• Bug Fix
• New Feature

Linked Issue

Related Issue: Add Google login via propel auth

Make sure you have

• Pulled from the default branch
• Documented your changes
• Linked the Issue
• Appointed a reviewer (if any)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add Google login via propel auth</issue_title>
<issue_description>Thinking about the best practices with propel auth. We already have slack login. I would like to add the ability to log in with Google as well. Please make all changes as a senior software engineer after you think about it step by step</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Google login via propel auth #177

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