feat(security): add JWT startup guard, configurable CORS, and externalize DB credentials

- Add check_jwt_secret() that refuses default JWT secret when APP_ENV != development
- Make CORS origins configurable via CORS_ORIGINS env var (comma-separated)
- Replace hardcoded postgres credentials in docker-compose.yml with env var references
- Add APP_ENV and cors_origins to config.py
- Update .env.example with all required variables and documentation
- Add tests for JWT startup guard and CORS configuration

Closes leeworks-agents/SPARC#4
Closes leeworks-agents/SPARC#5
Closes leeworks-agents/SPARC#6

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -1,21 +1,42 @@
 # SPARC Configuration
 
+# ---- Application Environment ----
+# Set to "production" or "staging" in deployed environments.
+# The API will refuse to start with the default JWT secret unless APP_ENV=development.
+APP_ENV=development
+
+# ---- API Keys ----
+
 # SerpAPI key for patent search
 API_KEY=your_serpapi_key_here
 
 # OpenRouter API key for LLM analysis
 OPENROUTER_API_KEY=your_openrouter_key_here
 
-# Database configuration
-# All messages are stored in the database for persistence and caching
-DATABASE_URL=postgresql://postgres:postgres@localhost:5432/sparc
+# ---- Database ----
 
-# Cache configuration
-# When USE_CACHE=true: check database for cached responses before making API calls
-# When USE_CACHE=false: always make fresh API calls (still stores results in database)
-# Default: true
-USE_CACHE=true
+# PostgreSQL credentials (used by docker-compose)
+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=change-me-to-a-secure-password
+POSTGRES_DB=sparc
 
-# JWT Secret for authentication
+# Full database URL (must match the credentials above)
+DATABASE_URL=postgresql://postgres:change-me-to-a-secure-password@localhost:5432/sparc
+
+# ---- Authentication ----
+
+# JWT Secret for signing tokens
 # IMPORTANT: Change this to a secure random string in production
 JWT_SECRET=your-secure-jwt-secret-change-in-production
+
+# ---- CORS ----
+
+# Comma-separated list of allowed origins for CORS
+# Defaults to http://localhost:3000,http://localhost:5173 when unset
+# CORS_ORIGINS=https://sparc.example.com,https://app.example.com
+
+# ---- Cache ----
+
+# When USE_CACHE=true: check database for cached responses before making API calls
+# When USE_CACHE=false: always make fresh API calls (still stores results in database)
+USE_CACHE=true