Move Arch storage config to PolicyEngine

[MaxGhenis]

Summary

• prefer PolicyEngine-owned Supabase environment variables while retaining legacy Cosilico fallbacks
• standardize Arch on POLICYENGINE_SUPABASE_SERVICE_KEY as the primary elevated Supabase key name
• update R2/bootstrap docs and messages to reference the PolicyEngine Cloudflare account
• update Arch repository ownership docs and metadata to PolicyEngine

Supabase follow-up

• checked the shared credentials docs available through Google Drive; I did not find a PolicyEngine Supabase API/service key there
• found the existing PolicyEngine Supabase project policyengine-strategy (naalkftuqvyhjpvwlvnd), but Supabase reports it as INACTIVE and refuses restore because it has been paused for more than 90 days
• did not leave GitHub repo variables/secrets pointing to that dead project

Tests

• uv run ruff check db/supabase_client.py tests/test_supabase_client.py arch/artifacts.py
• uv run pytest tests/test_supabase_client.py tests/test_arch_namespace.py -q