chore(release): v1.28.27 — runs on CPUs without AVX2

The bundled Skia x64 build (document thumbnails, scanned-PDF rasterization)
uses AVX2 instructions; on an x86-64 host without them — Celeron/Atom-class
NAS CPUs, pre-2013 Xeons, VMs masking CPU flags — the first canvas call is an
uncatchable SIGILL that kills the process and crash-loops the container (live
incident: a Synology DS1520+, exit 132 ~18 s after every boot, reported with
kernel traps and the correct root cause in hand).

The renderer is now gated on a one-time CPU-feature check before the module
ever loads: linux/x64 requires the avx2 cpuinfo flag; arm64 and non-Linux are
unaffected; unreadable cpuinfo fails open. Behind a closed gate, thumbnail
generation and PDF rasterization take their existing fail-soft paths (vault
shows type icons; PDFs read as text) and the boot-time thumbnail backfill
skips enqueueing. NATIVE_CANVAS=off|on overrides detection, documented in the
env example and whitelisted in compose.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.28.26
+  version: 1.28.27
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.