Fix PostgreSQL compatibility issues in CI and SQL profiling

Fixes #2185

- Add vendor detection to SQL panel profiling to prevent MySQL-specific
  commands on PostgreSQL
- Fix CI PostgreSQL job environment variables
- Fix malformed lint job strategy configuration
- Add comprehensive PostgreSQL compatibility tests

Changes:
- debug_toolbar/panels/sql/forms.py: Add vendor check in profile() method
- .github/workflows/test.yml: Add DB_* env vars for PostgreSQL, fix lint job
- tests/test_postgresql_compatibility.py: New test file for PostgreSQL fixes
- docs/changes.rst: Document PostgreSQL compatibility improvements

Author: abdibaker