feat(ci): add armv7 (ipq40xx) build target (#325)

[gnacho]

Adds an armv7 build for 32-bit ARMv7 routers (ipq40xx / arm_cortex-a7_neon-vfpv4), requested in #325.

• Workflow: ipq40xx/generic legs for both SDK generations (.ipk on 24.10, .apk on 25.12) plus a netgrip-linux-armv7 binary asset; cross-compile uses GOARM=7. The luci-app arch-all ipk skip now covers every non-arm64 leg.
• Self-update: an arm build resolves the netgrip-linux-armv7 asset.
• Heal: matches the -armv7.apk asset on 25.x. Also fixes a pre-existing heal bug where the asset URL grep was hardcoded to the .ipk naming, so apk routers never found a matching asset.
• install.sh: maps armv7l/armv7 to the new assets.
• README (EN/ES): requirements no longer say 64-bit ARM only.

The ipk was built and inspected locally with the ipq40xx/generic 24.10.5 SDK (Architecture field and embedded binary verified). End-to-end on real hardware is pending; the workflow_dispatch run for v0.72.2 publishes the first armv7 assets.

Closes #325