Add PostgreSQL compatibility matrix CI workflow

Introduced a GitHub Actions workflow to verify compatibility with PostgreSQL versions 14 through 18. Implemented a smoke test script to check core database metrics and version-specific differences. Updated the README with details about the compatibility matrix and usage instructions for the script.

README.md

@@ -2,6 +2,13 @@
 
 NexaPG is a Docker-based PostgreSQL monitoring platform for multiple remote targets, built with FastAPI + React.
 
+## PostgreSQL Version Support
+
+NexaPG targets PostgreSQL versions **14, 15, 16, 17, and 18**.
+
+- Compatibility is verified with an automated CI matrix.
+- Collector and overview queries include fallbacks for stats view differences between versions (for example `pg_stat_bgwriter` vs `pg_stat_checkpointer` fields).
+
 ## What it includes
 
 - Multi-target PostgreSQL monitoring (remote instances)
@@ -92,6 +99,13 @@ make logs      # follow logs
 make migrate   # run Alembic migrations in backend container
 ```
 
+PostgreSQL compatibility smoke check script:
+
+```bash
+PG_DSN='postgresql://postgres:postgres@127.0.0.1:5432/compatdb?sslmode=disable' \
+python backend/scripts/pg_compat_smoke.py
+```
+
 ## Environment variables reference
 
 All variables are defined in `.env.example`.
@@ -174,3 +188,11 @@ For production-like deployments behind HTTPS:
 - Set frontend API to relative path: `VITE_API_URL=/api/v1`
 - Route `/api/` from proxy to backend service
 - Keep frontend and API on the same public origin to avoid CORS/mixed-content problems
+
+## CI Compatibility Matrix
+
+GitHub Actions workflow:
+
+- `.github/workflows/pg-compat-matrix.yml`
+
+It runs smoke checks against PostgreSQL `14`, `15`, `16`, `17`, and `18`.