docs: add .env.example with required PostgreSQL env vars

Template documents all required env vars:
- AUTH_SECRET_KEY and ENCRYPTION_KEY (secrets)
- DATABASE_URL, AUTH_DATABASE_URL, TASKS_DATABASE_URL (PostgreSQL)
- ALLOWED_ORIGINS, SESSION_SECRET_KEY (optional)

After [SEC:C-4] removed DEV_MODE fallback, the .env must have
AUTH_DATABASE_URL set explicitly or the app crashes at startup.

backend/.env.example

@@ -0,0 +1,17 @@
+# ── Required: Secrets ──
+# Generate with: python -c "import secrets; print(secrets.token_urlsafe(32))"
+AUTH_SECRET_KEY=change-me-to-a-random-secret
+ENCRYPTION_KEY=change-me-to-a-fernet-key
+
+# ── Required: Database ──
+# PostgreSQL via docker-compose:
+DATABASE_URL=postgresql+psycopg2://postgres:postgres@localhost:5432/ss_tools
+AUTH_DATABASE_URL=postgresql+psycopg2://postgres:postgres@localhost:5432/ss_tools
+TASKS_DATABASE_URL=postgresql+psycopg2://postgres:postgres@localhost:5432/ss_tools
+
+# ── Optional ──
+ALLOWED_ORIGINS=http://localhost:5173
+SESSION_SECRET_KEY=change-me-to-a-random-secret
+FEATURES__DATASET_REVIEW=true
+FEATURES__HEALTH_MONITOR=true
+APP_TIMEZONE=Europe/Moscow