feat(server): load .env via dotenvy on startup

The binary now reads a .env file itself (dotenvy::dotenv() at the top of main),
so 'cargo run -p server' / the release binary pick up config without relying on
just's 'set dotenv-load'. Missing .env is a no-op.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -28,4 +28,5 @@ argon2 = "0.5"
 tower-sessions = "0.14"
 tower-sessions-sqlx-store = { version = "0.15", features = ["postgres"] }
 rpassword = "7"
+dotenvy = "0.15"
 memory-serve = "2.1"