fix: replace generic exit 1 with specific exit codes in ct/ and install/ scripts (#12475)

Part of #12467 — scripts only (no framework changes).

New exit codes 250-254 registered in api.func and error_handler.func:
- 250: App download failed or version not determined
- 251: App file extraction failed (corrupt/incomplete archive)
- 252: App required file or resource not found
- 253: App data migration required — update aborted
- 254: App user declined prompt or input timed out

Existing codes reused where applicable:
- 10: privileged/Docker required (unifi-os-server)
- 64: invalid user input (postgresql, tomcat)
- 71: system error (pulse useradd)
- 150: service failed to start (docker, npmplus)
- 153: build failed (booklore)
- 233: app not installed (evcc, endurain, grafana, loki, itsm-ng)
- 236: hardware not detected (unifi-os-server /dev/net/tun)
- 238: OS not supported (frigate)

misc/api.func

@@ -134,6 +134,7 @@ detect_repo_source
 #   * Proxmox custom codes (200-231)
 #   * Tools & Addon Scripts (232-238)
 #   * Node.js/npm errors (239, 243, 245-249)
+#   * Application Install/Update errors (250-254)
 # - Returns description string for given exit code
 # ------------------------------------------------------------------------------
 explain_exit_code() {
@@ -322,6 +323,13 @@ explain_exit_code() {
   248) echo "Node.js: Invalid C++ addon / N-API failure" ;;
   249) echo "npm/pnpm/yarn: Unknown fatal error" ;;
 
+  # --- Application Install/Update Errors (250-254) ---
+  250) echo "App: Download failed or version not determined" ;;
+  251) echo "App: File extraction failed (corrupt or incomplete archive)" ;;
+  252) echo "App: Required file or resource not found" ;;
+  253) echo "App: Data migration required — update aborted" ;;
+  254) echo "App: User declined prompt or input timed out" ;;
+
   # --- DPKG ---
   255) echo "DPKG: Fatal internal error" ;;