Remove operator auth — trust the LAN

Can't log in from a fresh LXC deploy, and the service is LAN-only by
design. Rip out the whole bcrypt-password / signed-cookie session
layer: internal/auth, login templates, gen-admin-password binary +
Makefile targets, auth config block, login/logout routes and the
RequireSession middleware wrap. Agent bearer-token auth on
/api/v1/runs/{id}/* is untouched.

Operators who want a password can front the service with a reverse
proxy — noted in README and docs/operations.md.

deploy/vetting.production.yaml

@@ -28,15 +28,6 @@ janitor:
   # Interval between cleanup sweeps. 0 defaults to 60.
   interval_minutes: 60
 
-auth:
-  # bcrypt hash of your admin password.
-  # Generate via: gen-admin-password 'your-password'
-  admin_password_bcrypt: "$2a$10$REPLACE_ME_WITH_A_REAL_BCRYPT_HASH_0123456789abcdefABCDEFxx"
-  # Random 32-byte hex string used to sign session cookies.
-  # Generate via: openssl rand -hex 32
-  session_secret_hex: "0000000000000000000000000000000000000000000000000000000000000000"
-  session_ttl_hours: 24
-
 dispatcher:
   max_concurrent_runs: 3