feat(operation): X-Just-Provisioned + X-Claimed handshake

Closes the sell-to-friend gap where a buyer's freshly-reset device
would briefly display the seller's photos before the buyer reached
/setup/{mac} to claim. The firmware had no way to tell the server
"I just got reset" — now it does.

Flow:
  - WiFi-setup completion (handle_connect in main.cpp) writes
    NVS_KEY_JUST_PROVISIONED=1 alongside the SSID/PASS save.
  - Every poll while the flag is set sends X-Just-Provisioned: 1.
  - Server (DeviceImageController, paired commit on the webApp side)
    responds with 204 + X-Interval-Ms when the binding is stale,
    forcing the device to its setup-QR fallback. Once the user
    re-claims via /setup/{mac}, the binding is fresh, and the server
    answers with X-Claimed: 1 alongside whatever response code applies.
  - Firmware clears the NVS flag on seeing X-Claimed: 1 — once
    cleared, the device is back to normal long-stable polling.

Tests:
  - PROV-A: flag set in NVS → header on the request
  - PROV-B: no flag → no header (steady state)
  - PROV-C: response with X-Claimed: 1 → flag cleared
  - PROV-D: response without X-Claimed → flag stays (so the next
    poll keeps signaling "not yet acknowledged")

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

test/mocks/config.h

@@ -7,6 +7,7 @@
 #define NVS_KEY_PASS        "pass"
 #define NVS_KEY_IMG_ID      "img_id"
 #define NVS_KEY_DRAW_NEEDED "draw"
+#define NVS_KEY_JUST_PROVISIONED "just_prov"
 #define IMAGE_PATH        "/img.bin"
 #define FETCH_INTERVAL_MS_FALLBACK 60000ULL
 #define SLEEP_CLAMP_MIN_MS         30000ULL