Security: Address Supabase security advisor warnings

## Security Warnings from Supabase Advisors

Our Supabase project has identified several security warnings that need to be addressed:

### 1. Function Search Path Mutable (2 instances)

**Affected Functions:**
- `public.search_similar_posts`
- `public.update_updated_at_column`

**Issue:** These functions have a role mutable search_path, which can be a security risk.

**Solution:** Set an explicit search_path parameter for these functions to prevent potential security vulnerabilities.

**Reference:** [Supabase Database Linter - Function Search Path](https://supabase.com/docs/guides/database/database-linter?lint=0011_function_search_path_mutable)

### 2. Leaked Password Protection Disabled

**Issue:** Password protection against leaked/compromised passwords is currently disabled.

**Solution:** Enable leaked password protection in Supabase Auth to check passwords against HaveIBeenPwned.org database.

**Reference:** [Supabase Auth - Password Security](https://supabase.com/docs/guides/auth/password-security#password-strength-and-leaked-password-protection)

## Action Items

- [ ] Add explicit search_path to `search_similar_posts` function
- [ ] Add explicit search_path to `update_updated_at_column` function
- [ ] Enable leaked password protection in Supabase Auth settings
- [ ] Run security advisors check after fixes to confirm resolution

## Priority
These are security-related warnings and should be addressed with high priority.

## Additional Context
These warnings were identified using `mcp__supabase__get_advisors` with type "security".