Offline bundle: pin and hash the webapp wheels, refuse unpinned ones, check for updates

- webapp/requirements.txt pins all eight wheels with sha256, at the fixed
  releases (lxml 6.1.3 for GHSA-vfmq-68hx-4jfw / PYSEC-2026-87, werkzeug
  3.1.9 for GHSA-g6x2-hccm-hh4m).
- download-packages.sh fetches exactly that set for Python 3.12 / x86_64
  into an emptied folder; PIP_DIR is honoured inside the container (own
  mount), files are handed back to the calling user, USE_DOCKER=1 forces
  the container on a noble host, an unknown package name now prints apt's
  error instead of failing silently, and linux-modules-extra-7.0.* is gone
  (it does not exist; 7.0 ships those modules in linux-modules).
- Playbook: the wheel installer refuses any wheel whose sha256 is not
  pinned, needs exactly one wheel per pin, and removes an older installed
  version by its RECORD first. The webapp copy is a full rsync every run
  (was cp with creates: app.py, which never updated an install).
- scripts/check-updates.py: offline debs against noble security/updates
  and the pins against PyPI + osv.dev; exit 1 when something needs
  updating. Run it before building a USB.

Tested: installer in a Python 3.12 venv (clean install, upgrade over lxml
6.0.4 + werkzeug 3.1.8 leaves no old dist-info, re-run is a no-op, a stray
wheel is refused); check-updates.py flags the April bundle (94 security
updates) and passes the pins.

webapp/requirements.txt

@@ -1,2 +1,24 @@
-flask
-lxml
+# Webapp Python dependencies, pinned with sha256 for the PXE server's
+# Python 3.12 (Ubuntu 24.04, x86_64). scripts/download-packages.sh fetches
+# exactly these into pip-wheels/; the playbook refuses any other wheel.
+# To update: change a version here, regenerate the hashes
+# (pip hash, or the PyPI JSON API), re-run download-packages.sh, and check
+# for known vulnerabilities (osv.dev) before building a USB.
+
+blinker==1.9.0 \
+    --hash=sha256:ba0efaa9080b619ff2f3459d1d500c57bddea4a6b424b60a91141db6fd2f08bc
+click==8.5.0 \
+    --hash=sha256:255bc9599cf7748b4b1a446ccc735421bd08a2ae529a8b88597d3de5664ee360
+flask==3.1.3 \
+    --hash=sha256:f4bcbefc124291925f1a26446da31a5178f9483862233b23c0c96a20701f670c
+itsdangerous==2.2.0 \
+    --hash=sha256:c6242fc49e35958c8b15141343aa660db5fc54d4f13a1db01a3f5891b98700ef
+jinja2==3.1.6 \
+    --hash=sha256:85ece4451f492d0c13c5dd7c13a64681a86afae63a5f347908daf103ce6d2f67
+lxml==6.1.3 \
+    --hash=sha256:379f8a75cf6eb7eef0af074b55f49ab73b868388a98de14646abcdfa4564bb11 \
+    --hash=sha256:909f4e927bb051f7740d6367285fc60cdcfdaf0258c2dba4ff5ba7eadadc250c
+markupsafe==3.0.4 \
+    --hash=sha256:8e124f974786f831d6043728e38296969d3579db8896fe004682f5758e613581
+werkzeug==3.1.9 \
+    --hash=sha256:6392e50c78460ba618e5b21f08a71f59c99ce99cdc6cf6e3dd7e6ccca8754fab