Sanitize envs, optimize Dockerfile, Turnstile

Redact sensitive AWS keys in backend/.env.example and add VITE_TURNSTILE_SITE_KEY and VITE_USE_TURNSTILE to frontend/.env.example. Optimize backend Dockerfile by installing Python requirements immediately after copying requirements.txt (enables Docker cache) and remove the duplicate later install. Update turnstile verification to skip checks when USE_SSL is disabled (dev/non-HTTPS) and keep the existing no-secret bypass; add debug logging for the SSL bypass.

backend/Dockerfile

@@ -18,6 +18,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
 COPY . .
 
 # Trust Windows/corporate root CAs if present (optional, no-op when certs/ is absent)
@@ -26,6 +28,4 @@ RUN test -f certs/windows-ca-bundle.crt \
     && update-ca-certificates \
     || true
 
-RUN pip install --no-cache-dir -r requirements.txt
-
 EXPOSE 8000