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, 9 insertions, 0 deletions

diff --git a/main/display.h b/main/display.h
index 58e71b7..e26845f 100644
--- a/main/display.h
+++ b/main/display.h
@@ -28,3 +28,12 @@ esp_err_t display_fill(uint16_t rgb565);
 
 // Show a deterministic test pattern (vertical color bars). M3 acceptance.
 esp_err_t display_test_pattern(void);
+
+// Blit an RGB565 source image to the panel, applying the current
+// orientation. Source dimensions must match the effective resolution:
+//   portrait  -> src is 480x800 (rotated 90° CW into the 800x480 panel)
+//   landscape -> src is 800x480 (copied 1:1)
+// Used by /frame after JPEG decode.
+esp_err_t display_present_rgb565(const uint16_t *src,
+                                 uint16_t        src_w,
+                                 uint16_t        src_h);