docs: add Supabase RLS recursion troubleshooting note

🎯 What needs to be done

Add a troubleshooting note in docs/TROUBLESHOOTING.md under database / Supabase configuration explaining how to resolve PostgreSQL error 42P17 (infinite recursion detected in policy for relation).

Why this matters

Contributors writing new Row-Level Security (RLS) policies may accidentally query the same table inside a subquery. Documenting the fix (SECURITY DEFINER functions or direct column lookups) prevents confusion and onboarding friction.

📍 Where to work

docs/TROUBLESHOOTING.md

🧭 Implementation guidance

1. Open docs/TROUBLESHOOTING.md.
2. Locate the database or Supabase troubleshooting section (or create subsection 3.3 Supabase RLS Policy Infinite Recursion (42P17)).
3. Add a concise problem explanation, cause, and recommended solution.
4. Save the file.

✅ Acceptance criteria

• docs/TROUBLESHOOTING.md documents PostgreSQL error 42P17 and its resolution clearly.
• Markdown link validation passes with 0 broken links.

🧪 Verification

Open docs/TROUBLESHOOTING.md and locate the database or Supabase configuration troubleshooting section. Add a concise explanation of PostgreSQL error 42P17, its RLS recursion cause, and the recommended SECURITY DEFINER or direct-column-lookup resolution. Verify that Markdown link validation passes with 0 broken links.