fix(espidf): enable mbedTLS PSK so ssl_client.cpp links

arduino-esp32 v2.0.17's libraries/WiFiClientSecure/src/ssl_client.cpp:23
wraps its entire body in:

  #if !defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED) \
   && !defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
  #  warning "Please call idf.py menuconfig ..."
  #else
    ssl_init / start_ssl_client / stop_ssl_socket /
    send_ssl_data / get_ssl_receive / data_to_read
  #endif

Our esp-idf-template/sdkconfig.defaults did not enable any PSK key-exchange
mode, so MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED was never auto-set by
mbedtls and ssl_client.cpp compiled to an empty translation unit. The
companion WiFiClientSecure.cpp still compiled and ended up in
libarduino-esp32.a with dangling references, breaking the link of every
sketch that pulls in HTTPClient or WiFiClientSecure (directly or
transitively).

Reproduced against the user's WiFi + HTTPClient example.com sketch on the
prod server and again locally with ESP-IDF v4.4.7 + arduino-esp32 v2.0.17;
the prebuilt sdkconfig that ships with arduino-esp32 itself sets both
flags, so we just align with that.

After the fix the same sketch links cleanly:
  velxio-sketch.bin binary size 0xbf470 bytes ... 25% free

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/esp-idf-template/sdkconfig.defaults

@@ -25,6 +25,15 @@ CONFIG_COMPILER_OPTIMIZATION_SIZE=y
 # WiFi
 CONFIG_ESP_WIFI_ENABLED=y
 
+# mbedTLS — PSK ciphersuites must be enabled or arduino-esp32 v2.0.17's
+# ssl_client.cpp compiles to an empty translation unit (its body is wrapped
+# in `#if !defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED) ... #else ... #endif`).
+# Without these, any sketch that pulls in HTTPClient or WiFiClientSecure
+# fails to link with undefined references to ssl_init / start_ssl_client /
+# stop_ssl_socket / send_ssl_data / get_ssl_receive / data_to_read.
+CONFIG_MBEDTLS_PSK_MODES=y
+CONFIG_MBEDTLS_KEY_EXCHANGE_PSK=y
+
 # LWIP — reassembly needed for slirp networking
 CONFIG_LWIP_IP4_REASSEMBLY=y
 CONFIG_LWIP_IP6_REASSEMBLY=y