feat: add environment variable validation on startup

- Created validateEnv utility for comprehensive environment validation
  - Validates required variables: JWT_SECRET, COUCHDB_URL, COUCHDB_DB_NAME
  - Validates optional variables with defaults: NODE_ENV, PORT, FRONTEND_URL
  - Enforces JWT_SECRET minimum length of 32 characters for security
  - Validates URL formats for COUCHDB_URL and FRONTEND_URL
  - Validates CouchDB database name format
  - Warns about missing optional services in production

- Integrated validation into server startup
  - Server exits with clear error messages if configuration is invalid
  - Logs environment configuration on startup (masks sensitive values)

- Updated test setup
  - Set proper 32+ character JWT_SECRET for tests
  - Added all required environment variables for validation

Security Benefits:
- Prevents server from starting with weak or missing credentials
- Catches configuration errors early before database connections
- Provides clear guidance on required variables
- Protects against default/example credentials in production

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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/server.js

@@ -12,6 +12,17 @@ const { errorHandler } = require("./middleware/errorHandler");
 const socketAuth = require("./middleware/socketAuth");
 const requestLogger = require("./middleware/requestLogger");
 const logger = require("./utils/logger");
+const { validateEnv, logEnvConfig } = require("./utils/validateEnv");
+
+// Validate environment variables before starting server
+try {
+  validateEnv();
+  logEnvConfig();
+} catch (error) {
+  logger.error('Environment validation failed:');
+  logger.error(error.message);
+  process.exit(1);
+}
 
 const app = express();
 const server = http.createServer(app);