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.

server/app/photo_queue.py

@@ -1,4 +1,5 @@
-"""Tracks which photo is currently displayed and what's queued up next.
+"""Tracks which photo is currently displayed, what's queued up next, and
+what's already been shown.
 
 `current_asset_id` only ever changes two ways: the configured refresh
 interval elapsing (`get_current`, called on every `GET /frame/image` --
@@ -12,6 +13,13 @@ rearrange, topped up (or trimmed) automatically to match
 `cfg.queue_target_len` (user-configurable from the web UI) as it's
 consumed. `queue_cursor` is separate, internal-only bookkeeping for where
 sequential top-up resumes in the album -- not shown or reordered in the UI.
+
+`history` is the mirror image of `queue`: every time `advance_forced`
+actually changes `current_asset_id`, the old one is pushed onto
+`history`. `back_forced` (the back-photo button) is the exact reverse of
+`advance_forced` -- it pops `history` back into `current_asset_id` and
+pushes the photo it's replacing onto the *front* of `queue`, so pressing
+next afterwards lands you right back where you were.
 """
 
 from __future__ import annotations
@@ -21,6 +29,8 @@ import time
 
 from .config import FrameConfig
 
+HISTORY_MAX_LEN = 20
+
 
 def _top_up(cfg: FrameConfig, assets: list[dict]) -> None:
     valid_ids = {a["id"] for a in assets}
@@ -69,8 +79,16 @@ def _top_up(cfg: FrameConfig, assets: list[dict]) -> None:
 
 def advance_forced(cfg: FrameConfig, assets: list[dict]) -> None:
     """Unconditionally moves to the next photo, ignoring elapsed time, and
-    resets the interval clock from now. Used only by the explicit
-    next-photo action (POST /frame/advance) -- always mutates cfg."""
+    resets the interval clock from now. Used by the explicit next-photo
+    action (POST /frame/advance) and by get_current() once the refresh
+    interval has elapsed -- always mutates cfg."""
+    if cfg.current_asset_id:
+        # Recorded regardless of *why* this advance happened (a manual
+        # next-press or the timer just elapsing) -- back should be able
+        # to undo either kind.
+        cfg.history.append(cfg.current_asset_id)
+        cfg.history = cfg.history[-HISTORY_MAX_LEN:]
+
     _top_up(cfg, assets)
     if cfg.queue:
         cfg.current_asset_id = cfg.queue.pop(0)
@@ -86,6 +104,29 @@ def advance_forced(cfg: FrameConfig, assets: list[dict]) -> None:
     _top_up(cfg, assets)
 
 
+def back_forced(cfg: FrameConfig, assets: list[dict]) -> bool:
+    """Unconditionally moves to the previously-current photo, the mirror
+    image of advance_forced() -- pops the most recent entry off history,
+    pushes the photo it's replacing onto the front of queue (so pressing
+    next afterwards returns to it), and resets the interval clock from
+    now. Skips over any history entries no longer in the album (deleted
+    since). Returns whether it actually moved -- False (history empty or
+    entirely stale) is a no-op, callers should still just display
+    whatever's current rather than treating it as an error. Used by the
+    back-photo button (POST /frame/back)."""
+    valid_ids = {a["id"] for a in assets}
+    while cfg.history:
+        previous_id = cfg.history.pop()
+        if previous_id not in valid_ids:
+            continue
+        if cfg.current_asset_id:
+            cfg.queue.insert(0, cfg.current_asset_id)
+        cfg.current_asset_id = previous_id
+        cfg.current_asset_set_at = time.time()
+        return True
+    return False
+
+
 def sync_queue_length(cfg: FrameConfig, assets: list[dict]) -> None:
     """Tops up or trims cfg.queue to match cfg.queue_target_len without
     otherwise touching current_asset_id. Used by GET /api/queue so a