fix(db): strip whitespace from DATABASE_URL to prevent SQLAlchemy crash at startup

[kapil971390]

What

_database_url() in Backend/db.py used a bare truthiness check on the value returned by os.getenv("DATABASE_URL"). In Python a whitespace-only string (" ", "\t", "\n") is truthy, so the guard passed that raw value straight into create_engine() — which immediately crashes with an invalid connection URL.

Root cause

# Before
if database_url:          # "   " passes this check
    return database_url   # returns "   " → SQLAlchemy crash

Fix

# After
if database_url and database_url.strip():
    return database_url.strip()

Both the guard and the return value now strip surrounding whitespace, so any whitespace-only DATABASE_URL falls through to the safe SQLite default.

Edge cases verified

DATABASE_URL value	Before fix	After fix
" " (spaces)	truthy → crash	stripped → empty → SQLite fallback ✓
"\t" (tab)	truthy → crash	stripped → empty → SQLite fallback ✓
"\n" (newline)	truthy → crash	stripped → empty → SQLite fallback ✓
"" (empty string)	falsy → SQLite fallback ✓	unchanged ✓
unset (None)	falsy → SQLite fallback ✓	unchanged ✓
valid URL	returned as-is ✓	returned stripped ✓

Impact

• No behavior change for correctly set DATABASE_URL values
• Prevents silent crash when DATABASE_URL is accidentally set to whitespace (e.g. trailing space in .env)