M5: POST /frame — hardware JPEG decode + orientation-aware paint

jpeg_decoder.{h,c} wraps esp_driver_jpeg (ESP32-P4 hardware decoder).
- One-time engine + DMA-aligned PSRAM scratch buffer setup (1 MB input,
  768 KB output @ panel native 800x480 RGB565).
- try_lock + unlock so concurrent /frame POSTs get 503 instead of
  queueing, per spec.
- jpeg_decoder_get_info() reports the dimensions; the http handler
  validates them against the effective resolution before painting.

display.h adds display_present_rgb565(src, w, h):
- Landscape: src is 800x480, memcpy 1:1 into the panel framebuffer.
- Portrait: src is 480x800, software rotate 90° CW into the 800x480
  panel framebuffer. (PPA / 2D-DMA hardware rotation is a later
  optimization if portrait latency matters.)

http_api.c adds POST /frame:
- Content-Type must be image/jpeg → else 400.
- Empty body → 400. > 1 MB → 413. Display not initialized → 500.
- jpeg_decoder_try_lock(0) for concurrency: second post returns 503.
- Body streamed into the decoder's input buffer in chunks.
- Decode failure or dimension mismatch → 400 + decode_errors++.
- Paint failure → 500.
- Success → frames_received++, last_frame_us = esp_timer_get_time(),
  204 No Content.

app_main initializes the JPEG decoder after display_init(). Both are
best-effort: failures log a warning and leave the rest of the firmware
running.

CMakeLists.txt: add esp_driver_jpeg to REQUIRES.

Known gap (M6 closes it): /frame currently paints regardless of
wake/sleep state. The 409-when-asleep rule lands with POST /state
in M6.

Build clean against ESP-IDF 5.4 (binary ~640 KB).

TESTING.md M5 expanded with portrait/landscape test commands,
ImageMagick test-image recipes, the full negative matrix (wrong
Content-Type, oversize, wrong dims, concurrent, garbage), and the
M6 dependency note.

1 files changed, 7 insertions, 1 deletions

diff --git a/main/app_main.c b/main/app_main.c
index 316f702..4ac925d 100644
--- a/main/app_main.c
+++ b/main/app_main.c
@@ -1,5 +1,6 @@
 #include "display.h"
 #include "http_api.h"
+#include "jpeg_decoder.h"
 #include "mdns_service.h"
 #include "net_eth.h"
 #include "nvs_config.h"
@@ -41,7 +42,12 @@ void app_main(void)
         ESP_LOGW(TAG, "display init failed — continuing without panel");
     }
 
-    // TODO M5: /frame JPEG decode -> framebuffer
+    // JPEG decoder is the M5 dependency for POST /frame. Best-effort:
+    // if it fails (e.g. hardware not present in sim) /frame just returns 500.
+    if (jpeg_decoder_init() != ESP_OK) {
+        ESP_LOGW(TAG, "jpeg decoder init failed — /frame will be unavailable");
+    }
+
     // TODO M6: POST /state + idle timer
     // TODO M7: Capacitive touch -> outbound /state POST
     // TODO M8: Local screens (IP, loading) + BOOT button