Add FastAPI server: pulls from Immich, pre-processes for the panel

Implements the server side of the architecture decided on: the ESP32-C6
has no PSRAM and a tight RAM budget, so all the heavy lifting (JPEG
decode, resize, Floyd-Steinberg dithering, 6-color quantization, 4bpp
packing) happens here instead of on-device. The frame just does a single
GET and streams the response straight to SPI.

- GET /frame/image: looks up the current cursor's asset in the configured
  Immich album, downloads its preview thumbnail, and returns it packed
  into the panel's exact 800x480/4bpp/2px-per-byte format
  (application/octet-stream, always exactly 192,000 bytes).
- GET / + POST /api/config + GET /api/albums: a small web UI for entering
  the Immich URL/API key and picking an album, rather than cramming that
  into the ESP32's captive portal form.
- Config (Immich creds, selected album, cursor) persists to a JSON file
  via a docker-compose volume mount.

Verified locally with a venv (Docker isn't available in this environment):
unit-tested image_pipeline against a synthetic image (exact byte count,
valid panel color codes only), and ran a full end-to-end pass against a
mock Immich HTTP server exercising the real /frame/image path.

Pinned dependency versions in requirements.txt after hitting a real bug
with unpinned floors: the latest starlette (1.3.1) resolved by `pip
install fastapi` breaks Jinja2Templates outright.

Not yet wired to the ESP32 side (task 6) or authenticated -- /frame/image
is unauthenticated for now, fine on a trusted LAN but worth revisiting
once the firmware sends a shared device token.

server/README.md

@@ -0,0 +1,51 @@
+# ESPresso Frame Server
+
+Pulls photos from an [Immich](https://immich.app) album, resizes/dithers/quantizes
+them to the E Ink Spectra 6 panel's exact 6-color format, and serves the
+frame a ready-to-display image once an hour. All the image processing
+happens here so the ESP32 never has to decode a JPEG or run a dithering
+algorithm itself -- it just streams the response straight to the panel.
+
+## Setup
+
+1. **Get an Immich API key**: in Immich, go to Account Settings -> API Keys
+   -> New API Key. Read-only access to albums/assets is enough.
+2. **Run the server**:
+   ```
+   docker compose up -d
+   ```
+3. Open `http://<this-machine>:8420/` in a browser, enter your Immich URL
+   and API key, click **Load Albums**, pick one, and **Save**.
+4. On the ESP32's captive portal setup form, set the **Tools Server** field
+   to `<this-machine>:8420`.
+
+## Endpoints
+
+- `GET /` -- config UI
+- `GET /api/albums` -- lists Immich albums (used by the config UI)
+- `POST /api/config` -- saves Immich URL/API key/album/order
+- `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)
+- `GET /health` -- liveness check
+
+## Notes
+
+- Config (including the Immich API key) is stored in `./data/config.json`
+  on the host via the compose volume mount.
+- `/frame/image` isn't authenticated yet. That's fine on a trusted home
+  LAN for now, but worth revisiting once the ESP32 side is wired up to
+  send a shared device token.
+- The 6-color palette RGB values in `app/image_pipeline.py` are
+  approximations, not measured values (Waveshare doesn't publish exact
+  color primaries for this panel) -- tune them once you can compare a
+  rendered test image against the real panel.
+
+## Local development (without Docker)
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+CONFIG_PATH=./data/config.json uvicorn app.main:app --reload --port 8420
+```