fix(install): add python3-dev for armv7l cffi build

[vpetersson]

Issues Fixed

32-bit Raspberry Pi OS (armv7l) installs of v2026.6.x abort while building cffi with:

fatal error: Python.h: No such file or directory

Reported on the forum: https://forums.screenly.io/t/anthias-v2026-6-3-installation-stops-halfway-on-pi3-b/6716

Description

bin/install.sh uses uv, which normally manages its own Python interpreter (headers included). But python-build-standalone has no armv7l build, so on 32-bit Pi OS uv falls back to the system python3. There is also no prebuilt cffi wheel for armv7, so uv compiles it from sdist — which needs the CPython development headers (Python.h). The apt list already had libffi-dev and libssl-dev (cffi's C-lib deps) but not the Python headers.

Adds python3-dev to the apt install list. 64-bit/arm64/x86 installs use uv's managed Python and prebuilt wheels, so they never compile cffi — installing the package there is harmless and keeps the path uniform.

Checklist

• I have performed a self-review of my own code.
• New and existing unit tests pass locally and on CI with my changes.
• I have done an end-to-end test for Raspberry Pi devices.
• I have tested my changes for x86 devices.
• I added a documentation for the changes I have made (when necessary).

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vpetersson]

Validated on native armv7 (32-bit) hardware

Ran the exact install.sh Python step — uv sync --no-default-groups --group host --no-install-project (uv pinned 0.9.17) — inside a native arm32v7/debian:trixie container on a Pi 4 host (arm64 kernel, 4 KB pages, so armv7 runs without qemu). Trixie ships Python 3.13, matching .python-version, so uv falls back to the system interpreter exactly as it does on a 32-bit Pi 3.

Before (no python3-dev) — reproduces the forum failure verbatim:

arm-linux-gnueabihf-gcc ... -I/usr/include/python3.13 -c src/c/_cffi_backend.c ...
src/c/_cffi_backend.c:2:10: fatal error: Python.h: No such file or directory
help: `cffi` (v2.0.0) was included because `anthias:host` depends on `ansible-core`
      which depends on `cryptography` which depends on `cffi`
=== EXIT CODE: 1 ===

After (with python3-dev from this PR) — succeeds:

Built cffi==2.0.0
Installed 17 packages  (+ cffi==2.0.0  + cryptography==49.0.0  + ansible-core==2.21.0 ...)
=== EXIT CODE: 0 ===
cffi 2.0.0 OK

Note: the Pi 5's default 2712 kernel uses 16 KB pages, which can't load 4 KB-page armhf userspace (libz.so.1: ELF load command ... not page-aligned), so the 32-bit validation was done on the Pi 4 host instead — still native armv7, no emulation.