chore: Update sensitive information in documentation and code to use environment variables

This commit replaces hardcoded sensitive information, such as database passwords and secret keys, in the README and deployment documentation with placeholders and instructions to use environment variables. This change enhances security by preventing exposure of sensitive data in the codebase. Additionally, it updates the database session management to raise an error if the DATABASE_URL environment variable is not set, ensuring proper configuration for development environments.

backend/db/session.py

@@ -20,8 +20,12 @@ def get_database_url() -> str:
     db_url = os.getenv("DATABASE_URL")
     if db_url:
         return db_url
-    # Default to PostgreSQL for development
-    return "postgresql+psycopg2://punimtag:punimtag_password@localhost:5432/punimtag"
+    # Default to PostgreSQL for development (without password - must be set via env var)
+    # This ensures no hardcoded passwords in the codebase
+    raise ValueError(
+        "DATABASE_URL environment variable not set. "
+        "Please set DATABASE_URL in your .env file or environment."
+    )
 
 
 def get_auth_database_url() -> str: