[P2] Include required database and encryption settings in .env.example #63

Recorded during review of main at a1eb305ccfa2f3fc8ddede4875765cf1b099bd2b. This issue records a finding only; no implementation changes were made or requested.

Finding

The example configuration omits DATABASE_URL and ENCRYPTION_KEY, although startup requires both. The README table lists them, but copying and filling the example does not produce a complete configuration.

Evidence

• .env.example:1
• src/config.rs:59

Desired outcome

Include both mandatory settings and concise format/generation guidance, keeping the example aligned with startup validation.