fix(deps): make pygobject an optional extra, not a hard dependency

pygobject ships no PyPI wheels, so pip install always compiles it from
source, which requires the OS gobject-introspection dev package at
install time. As a mandatory dependency this broke plain `pip install
csaxs_bec` on CI and on Read the Docs, neither of which has that
package installed. Move it to the `allied_vision` extra so only hosts
that actually run the camera (which already need Aravis + the OS
package present) opt in via `pip install csaxs_bec[allied_vision]`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

csaxs_bec/devices/allied_vision_cameras/README.md

@@ -29,11 +29,24 @@ needed).
   environment, same as the IDS camera tests run without `pyueye`.
 - **Talking to real hardware** — two things, both local to the machine running the device
   server (or wherever you instantiate `AlliedVisionAravisCamera`/`Camera` directly):
-  1. **PyGObject** (`pygobject`, the `gi` module) — already listed in `pyproject.toml`,
-     pinned to `<3.52`. It's the GObject Introspection bridge, not Aravis itself. The pin
-     matters: PyGObject 3.52+ requires `girepository-2.0` (GLib >=2.80 /
-     gobject-introspection >=1.80), which RHEL9's system packages don't provide (only the
-     older `girepository-1.0` via `gobject-introspection` 1.68) — `pip install pygobject`
+  1. **PyGObject** (`pygobject`, the `gi` module) — the GObject Introspection bridge, not
+     Aravis itself. It is **not** installed by a plain `pip install csaxs_bec` — it lives
+     in the `allied_vision` extra (`pyproject.toml`'s `[project.optional-dependencies]`),
+     pinned to `<3.52`:
+     ```bash
+     pip install csaxs_bec[allied_vision]
+     # or, into an existing environment:
+     pip install "pygobject<3.52"
+     ```
+     It's opt-in rather than a hard dependency because `pygobject` ships no PyPI wheels —
+     `pip install` always compiles it from source, which needs the OS
+     `gobject-introspection` dev package present *at install time* (see below). As a
+     mandatory dependency this broke plain `pip install csaxs_bec` on CI and on Read the
+     Docs, neither of which has that OS package (and RTD's build image isn't something we
+     can add packages to) — so install it explicitly only on hosts that actually run this
+     camera. The version pin matters too: PyGObject 3.52+ requires `girepository-2.0` (GLib
+     >=2.80 / gobject-introspection >=1.80), which RHEL9's system packages don't provide
+     (only the older `girepository-1.0` via `gobject-introspection` 1.68) — installing it
      unpinned fails at build time with `Dependency 'girepository-2.0' is required but not
      found`. Unpin once deployment hosts move to a newer GLib/gobject-introspection.
   2. **Aravis** — a native C library with a GObject Introspection typelib. It is *not* a
@@ -43,9 +56,11 @@ needed).
 
 ### Installing PyGObject + Aravis (Linux)
 
-`pygobject` alone (`pip install pygobject`, already in `pyproject.toml`) only gets you the
-`gi` bridge — `import gi; gi.require_version("Aravis", "0.8")` will raise until the Aravis
-library and its typelib are installed on the system.
+Install the OS Aravis/gobject-introspection packages **first** — `pip install
+csaxs_bec[allied_vision]` (or `pip install "pygobject<3.52"`) needs the
+`gobject-introspection-1.0` pkg-config file present to build, and even once installed,
+`import gi; gi.require_version("Aravis", "0.8")` will raise until the Aravis library and
+its typelib are also installed on the system.
 
 1. **Debian/Ubuntu** — Aravis and its GObject Introspection bindings are packaged directly:
    ```bash

pyproject.toml

@@ -22,12 +22,6 @@ dependencies = [
     "rich",
     "pyepics",
     "pyueye",                    # for the IDS uEye camera
-    "pygobject<3.52",             # for the AlliedVision Alvium camera (native Aravis/GenICam, no EPICS IOC)
-                                  # pinned below 3.52: that version switched to requiring
-                                  # girepository-2.0 (GLib >=2.80/gobject-introspection >=1.80),
-                                  # which RHEL9's system packages don't provide (only the older
-                                  # girepository-1.0 via gobject-introspection 1.68) -- unpin once
-                                  # deployment hosts are upgraded past that.
     "bec_widgets",
     "zmq",
     "opencv-python",
@@ -36,6 +30,22 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+allied_vision = [
+    "pygobject<3.52",             # for the AlliedVision Alvium camera (native Aravis/GenICam, no EPICS IOC)
+                                  # pinned below 3.52: that version switched to requiring
+                                  # girepository-2.0 (GLib >=2.80/gobject-introspection >=1.80),
+                                  # which RHEL9's system packages don't provide (only the older
+                                  # girepository-1.0 via gobject-introspection 1.68) -- unpin once
+                                  # deployment hosts are upgraded past that.
+                                  #
+                                  # Kept out of the default dependency set: pygobject ships no
+                                  # PyPI wheels, so `pip install` always builds it from source,
+                                  # which needs the OS gobject-introspection dev package present
+                                  # at *install* time. As a hard dependency this broke plain
+                                  # `pip install csaxs_bec` on CI and on Read the Docs (neither
+                                  # has that OS package, and RTD's build image isn't ours to
+                                  # change) -- see csaxs_bec/devices/allied_vision_cameras/README.md.
+]
 dev = [
     "black",
     "copier",