fix: Docker entrypoint arg handling + configurable model directory

Fixes #384: docker run with --source/--tick-ms flags now works correctly.
Fixes #399: model files in mounted volumes are now discoverable via MODELS_DIR env var.

Root cause (issue #384):
The Dockerfile used ENTRYPOINT ["/bin/sh", "-c"] with a shell-form CMD.
When users passed flags like `--source wifi --tick-ms 500` as docker run
arguments, Docker replaced CMD entirely, resulting in
`/bin/sh -c "--source wifi --tick-ms 500"` which executes `--source` as
a shell command → `--source: not found`.

Root cause (issue #399):
Model directory was hardcoded to the relative path `data/models`. When Docker
users mounted models to `/app/models/`, the scan looked in the wrong place.

Changes:

1. docker/docker-entrypoint.sh (new):
   - Proper entrypoint script that handles both env-var-based defaults and
     user-passed CLI flags
   - No arguments → starts server with CSI_SOURCE env var as --source
   - Flag arguments (start with -) → prepends /app/sensing-server + defaults,
     appends user flags (clap last-wins allows overrides)
   - Non-flag first arg → exec passthrough (e.g., /bin/sh for debugging)
   - Sets --bind-addr 0.0.0.0 (was 127.0.0.1 which blocks container access)

2. docker/Dockerfile.rust:
   - Switch from ENTRYPOINT ["/bin/sh", "-c"] to exec-form entrypoint
   - Add MODELS_DIR env var (default: data/models)
   - COPY the entrypoint script into the image

3. docker/docker-compose.yml:
   - Remove shell-form command (entrypoint handles defaults)
   - Add MODELS_DIR env var

4. model_manager.rs + main.rs:
   - Replace hardcoded `data/models` path with `effective_models_dir()`
     / `models_dir()` that reads MODELS_DIR env var at runtime
   - Docker users can now: docker run -v /host/models:/app/models -e MODELS_DIR=/app/models

5. tests/test_docker_entrypoint.sh (new, 17 tests):
   - Default CSI_SOURCE substitution (6 assertions)
   - Custom CSI_SOURCE propagation
   - User-passed flag arguments (--source, --tick-ms, --model)
   - Unset CSI_SOURCE defaults to auto
   - Explicit command passthrough
   - MODELS_DIR env var propagation

docker/docker-compose.yml

@@ -18,8 +18,13 @@ services:
       #          wifi           — use host Wi-Fi RSSI/scan data (Windows netsh)
       #          simulated      — generate synthetic CSI data (no hardware required)
       - CSI_SOURCE=${CSI_SOURCE:-auto}
-    # command is passed as arguments to ENTRYPOINT (/bin/sh -c), so $CSI_SOURCE is expanded by the shell.
-    command: ["/app/sensing-server --source ${CSI_SOURCE:-auto} --tick-ms 100 --ui-path /app/ui --http-port 3000 --ws-port 3001"]
+      # MODELS_DIR controls where the server scans for .rvf model files.
+      # Mount a host directory and set this to make models visible:
+      #   volumes: ["/path/to/models:/app/models"]
+      #   MODELS_DIR=/app/models
+      - MODELS_DIR=${MODELS_DIR:-data/models}
+    # No explicit command needed — docker-entrypoint.sh uses CSI_SOURCE.
+    # Override with: command: ["--source", "esp32", "--tick-ms", "500"]
 
   python-sensing:
     build: