Add back-photo button; consolidate reset/manage onto one hold-duration button

Back button (new GPIO0, POST /frame/back): the server now tracks a
bounded history of previously-current photos (photo_queue.py), pushed
to on every advance (auto or forced) and popped by back_forced() --
symmetric with advance, so pressing next afterwards returns to right
where you were. frame_client.c's force_advance bool becomes a 3-way
fetch_action_t (NORMAL/ADVANCE/BACK) threaded through the whole fetch
path.

Also folds the separate reset and manage buttons onto one pin
(combo_button.c, replacing reset_button.c/manage_button.c entirely),
disambiguated by hold duration: quick press shows the management menu
(unchanged), ~3s hold-then-release soft-resets (esp_restart(), config
kept -- new), ~15s hold factory-resets (today's old reset behavior,
extended from 10s for clearer tier separation). Driven by a production
board (Seeed XIAO ESP32-C6) exposing only 3 of the ESP32-C6's 8
deep-sleep-wakeup-capable GPIOs -- next/back keep their own dedicated
pins where instant response matters most, everything else shares the
third pin via timing instead of needing its own. Same three-pin layout
now works on both the dev board and the production board.

Fixed a fast-tap bug in combo_button_check() before shipping: it only
did a live gpio_get_level() read to decide whether the button was
pressed at all, so a press fast enough to already be released by the
time boot reached that check was missed entirely (treated as "never
pressed" rather than "quick press"). Added the same latched
esp_sleep_get_gpio_wakeup_status() check the other buttons already use
for exactly this reason.

firmware/main/back_button.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <stdbool.h>
+
+/**
+ * Configures the back-photo button GPIO (CONFIG_FRAME_BACK_BUTTON_GPIO,
+ * active-low with internal pull-up) and arms it as a deep-sleep wakeup
+ * source, same as next_button_init(). Call once, early in app_main(),
+ * before the device might enter deep sleep.
+ *
+ * A no-op if CONFIG_FRAME_BACK_BUTTON_GPIO is negative (button disabled).
+ */
+void back_button_init(void);
+
+/**
+ * Returns whether the back-photo button is currently held, debounced with
+ * a couple of short re-checks to reject noise. Same immediate-response
+ * reasoning as the next-photo button -- no long hold-to-confirm gate.
+ */
+bool back_button_check(void);