fix: resolve admin registration failure on fresh Docker installs (#81)

Three bugs caused 403 errors when creating the first admin on fresh
Docker deployments (Unraid, Komodo):

1. db.rs: Schema application failures were silently swallowed. The app
   started with no tables, causing all auth queries to fail. Now the
   startup aborts if schema cannot be applied, with a fallback
   statement-by-statement executor that handles dollar-quoted blocks.
   Retries increased to 5 with 2s intervals.

2. auth_application_service.rs: count_admin_users() used fragile string
   matching (contains "does not exist")) on multi-layer wrapped errors.
   count_all_users() rejected admin creation on any DB error. Both now
   allow admin creation on any error for bootstrap scenarios.

3. auth_handler.rs: Redundant 60-line handler-level admin detection
   duplicated service-layer logic and generated noisy ERROR logs on
   fresh installs. Removed entirely - service layer handles it all.

Closes #81

src/application/services/auth_application_service.rs

@@ -167,32 +167,18 @@ impl AuthApplicationService {
                                 tracing::info!("Allowing admin creation on clean install");
                             },
                             Err(e) => {
-                                tracing::error!("Error counting users: {}", e);
-                                // For security, if we cannot verify, we reject admin creation
-                                return Err(DomainError::new(
-                                    ErrorKind::AccessDenied,
-                                    "User",
-                                    "Creating additional admin users is not allowed"
-                                ));
+                                // Cannot verify user count — treat as bootstrap scenario
+                                tracing::warn!("Could not count users ({}). Allowing admin creation for bootstrap.", e);
                             }
                         }
                     }
                 },
                 Err(e) => {
-                    let err_msg = e.to_string();
-                    // If the table doesn't exist, this is a fresh install - allow admin creation
-                    if err_msg.contains("does not exist") || err_msg.contains("relation") {
-                        tracing::info!("Database tables not yet ready, treating as fresh install - allowing admin creation");
-                        // Continue with registration - this is a fresh install
-                    } else {
-                        tracing::error!("Error counting admin users: {}", e);
-                        // For security, if we cannot verify, we reject admin creation
-                        return Err(DomainError::new(
-                            ErrorKind::AccessDenied,
-                            "User",
-                            "Creating additional admin users is not allowed"
-                        ));
-                    }
+                    // Any DB error (table missing, connection issue, etc.) means we
+                    // cannot verify admin state. Allow admin creation so the user can
+                    // bootstrap the system. If the DB is truly broken the INSERT will
+                    // fail anyway with a clear error.
+                    tracing::warn!("Could not count admin users ({}). Allowing admin creation for bootstrap.", e);
                 }
             }
         }