refactor: remove GATEWAY_USER_TOKENS, fix review feedback

GATEWAY_USER_TOKENS never went to production — replaced entirely by
DB-backed user management via /api/admin/users and /api/tokens.

Removed:
- UserTokenConfig struct and GATEWAY_USER_TOKENS env var parsing
- user_tokens field from GatewayConfig
- GatewayChannel::new_multi_auth() constructor
- Env-var user migration block in main.rs (~90 lines)
- multi_tenant auto-detection from GATEWAY_USER_TOKENS (now runtime
  via db.has_any_users() in app.rs)

Review fixes (zmanian):
- User ID generation: UUID instead of display-name derivation (#1)
- Invitation accept moved to public router (no auth needed) (#3)
- libSQL get_invitation_by_hash aligned with postgres: filters
  status='pending' AND expires_at > now (#4)
- UUID parse: returns DatabaseError::Serialization instead of
  unwrap_or_default (#7)
- PostgreSQL SELECT * replaced with explicit column lists (#8)
- Sort order aligned (both backends use DESC) (#6)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

src/channels/web/handlers/invitations.rs

@@ -162,12 +162,7 @@ pub async fn invitations_accept_handler(
         return Err((StatusCode::GONE, "Invitation has expired".to_string()));
     }
 
-    // Generate a user id from the display name.
-    let new_user_id = display_name
-        .to_ascii_lowercase()
-        .split_whitespace()
-        .collect::<Vec<_>>()
-        .join("-");
+    let new_user_id = Uuid::new_v4().to_string();
 
     let now = chrono::Utc::now();
     let user_record = UserRecord {