Wire in SQLite DB; Have routes write / read from the db

[Howard-nolan]

Change Summary:

• Wired DB into handlers; shortened URLs now persist to SQLite and redirects fetch from DB.
• Added random code generation with collision retries, URL validation/normalization, and real redirects.
• Fixed goose embed/dialect setup and used request-aware DB calls.

How It Works:

• cmd/server/main.go opens ./data/app.db, runs migrations, builds a handlers.Handler with the shared *sql.DB, and registers routes.
• ShortenHandler validates/normalizes the incoming URL (adds https:// if missing), generates a 6-char code, inserts (code, url) into links via ExecContext with retries on collisions, and returns the short link.
• RedirectHandler looks up the URL by code using QueryRowContext; 404s if missing, otherwise issues an HTTP 302 to the stored URL.

Additional Notes (optional):

• Bare hostnames like www.google.com are accepted; malformed URLs still 400.