Add problem statement tooling, API enhancements, and volunteer email fix

[gregv]

Summary

• Problem statement CLI tooling: Added list_problem_statements action to scripts/util.py with --search keyword filtering, plus get_all_problem_statements() and get_all_nonprofits() in firebase utils
• Problem statement creation scripts: Added scripts/create_client_case_mgmt_problem_statement.py (linked to 6 nonprofits) and scripts/create_wial_problem_statement.py (linked to WIAL) for SRD-based problem statement provisioning
• API enhancements: GET /problem_statement/<id>/nonprofit reverse lookup endpoint; PATCH /hackathon/problem_statements to manage visible problem statements per hackathon
• Volunteer email wording: Updated confirmation email from "volunteering" to "applying" to better reflect the application/review process

Test plan

• Run python scripts/util.py list_problem_statements — verify it lists all problem statements
• Run python scripts/util.py list_problem_statements --search "NMTSA" — verify filtering
• Test GET /api/messages/problem_statement/<id>/nonprofit returns associated nonprofits
• Test PATCH /api/messages/hackathon/problem_statements updates visible problem statements
• Verify volunteer signup email uses "applying" wording

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