Server: quiet hours (no wake overnight), zero firmware changes needed

Purely a server-side decision: GET /frame/config hands back a longer
refresh_interval_s while quiet hours are in effect (exactly the seconds
until they end), and clamps the normal interval so the device's next
wake lands at the boundary instead of wandering into the window, when
outside it but approaching. A device already mid-sleep when quiet hours
begin can still land one wake inside the window -- unavoidable without
touching the firmware, since it has no wall-clock awareness -- but from
that wake on it sleeps straight through to the end.

Window is "HH:MM"-"HH:MM", wrap-past-midnight aware (e.g. 22:00-07:00),
in the server's local timezone -- added tzdata to the Dockerfile since
python:3.12-slim doesn't include it and TZ would otherwise silently
resolve to nothing and fall back to UTC.

Also fixed the "overdue" device-status check to account for quiet hours:
without this it would falsely flag a device sleeping through a long
quiet window as unreachable.

server/README.md

@@ -51,7 +51,8 @@ algorithm itself -- it just streams the response straight to the panel.
   toggle, upcoming-photos count, now-displaying + drag-to-reorder
   upcoming grid -- not Immich URL/API key, see Setup above)
 - `GET /api/albums` -- lists Immich albums (used by the config UI)
-- `POST /api/config` -- saves album/order/orientation/refresh_interval_s/smart_crop_faces/queue_target_len.
+- `POST /api/config` -- saves
+  album/order/orientation/refresh_interval_s/smart_crop_faces/queue_target_len/quiet_hours_*.
   `orientation` (`landscape`, `portrait`, `landscape_flipped`,
   `portrait_flipped`) matches how the frame is physically hung: photos
   are composed/cropped for that shape (portrait crops at 480x800), then
@@ -60,7 +61,21 @@ algorithm itself -- it just streams the response straight to the panel.
   (QRs, text, battery indicator, face labels) still renders in native
   panel orientation, so on a portrait-hung frame it appears rotated
   90° to the viewer -- QR codes scan fine at any rotation, but the text
-  reads sideways. A known limitation, not planned to change soon
+  reads sideways. A known limitation, not planned to change soon.
+  `quiet_hours_enabled`/`quiet_hours_start`/`quiet_hours_end`
+  (`"HH:MM"`, may wrap past midnight, e.g. `22:00`-`07:00`) don't touch
+  the device at all -- purely a server decision about what
+  `refresh_interval_s` to hand back from `GET /frame/config` below,
+  computed in `_effective_refresh_interval_s`. Uses the server's local
+  timezone (`TZ` in `docker-compose.yml.example` -- the image needs
+  `tzdata` for a named zone to actually resolve, already installed in
+  the provided `Dockerfile`). The device can still land one wake right
+  at the start of the window (nothing server-side can prevent that
+  without touching the firmware, since the device doesn't know wall-clock
+  time), but from that wake on it's told to sleep exactly until the
+  window ends. The "overdue" indicator in `/api/queue`'s `device` object
+  also accounts for this -- it won't falsely flag a device that's
+  legitimately sleeping through a long quiet-hours window
 - `GET /frame/image` -- returns the current photo pre-processed into the
   panel's raw 800x480, 4-bit-per-pixel, 2-pixels-per-byte format
   (`application/octet-stream`, exactly 192,000 bytes). **Side-effect-free**