refactor: optimize docker, robust config & country handling

- optimize(docker): use distroless/python3-debian12 & multi-stage build
- feat(app): replace Gunicorn with Waitress (pure python)
- feat(countries): replace static list with pycountry (dynamic & complete)
- feat(config): enforce mandatory APPRISE_URL (fatal if missing)
- feat(logging): add structured logging and graceful shutdown signal handling
- docs: update README with required env var and correct usage

Dockerfile

@@ -1,12 +1,35 @@
-FROM python:3.11-alpine
+# ------------------------------------
+# Stage 1: Build dependencies
+# ------------------------------------
+FROM python:3.11-slim-bookworm AS builder
+
+WORKDIR /build
+COPY requirements.txt .
+
+# Install dependencies to a specific directory
+RUN pip install --no-cache-dir --target=/build/packages -r requirements.txt
+
+# ------------------------------------
+# Stage 2: Final distroless image
+# ------------------------------------
+# Using Debian 12 (Bookworm) based distroless to match builder
+FROM gcr.io/distroless/python3-debian12
 
 WORKDIR /app
 
-COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+# Add our packages to python path
+ENV PYTHONPATH=/app/packages
+ENV PYTHONDONTWRITEBYTECODE=1
+ENV PYTHONUNBUFFERED=1
 
+# Copy dependencies
+COPY --from=builder /build/packages /app/packages
+
+# Copy application code
 COPY app.py .
 
+# Expose port (metadata only, distroless doesn't enforce)
 EXPOSE 5000
 
-CMD ["gunicorn", "-b", "0.0.0.0:5000", "app:app"]
+# Run app.py directly (which uses waitress with signal handling)
+CMD ["app.py"]