Re-attempt server Docker image base bump (node 22 → 24/26)

[TerrifiedBug]

Context

The docker group bump (#401) moved the server image to node:26-alpine, but it was reverted to node:22-alpine (commit a248c331) because the Server Image build broke two ways on node 26:

1. corepack removed — node 25+ images no longer bundle corepack, so RUN corepack enable failed (corepack: not found, exit 127).
2. prisma generate schema corruption — Prisma 7.8's prisma generate mis-reads prisma/schema.prisma inside node:26-alpine (musl), emitting ~187 Error validating: This line is invalid... errors (the schema is parsed as garbage — only quote characters survive per line). The same Prisma 7.8 generate works fine on node:22-alpine and on the glibc CI runner.

The agent image (golang 1.22 → 1.26) and the alpine:3.23 vector stage were unaffected and remain bumped.

To upgrade later

• Prefer node:24-alpine (current LTS; still bundles corepack) as a smaller jump, or solve the node:26 issues:
  • Add RUN npm install -g corepack && corepack enable (corepack is still on npm), and
  • Root-cause the Prisma 7.8 schema mis-read on node 26 + Alpine/musl (likely a Prisma engine / Node 26 interaction; may be fixed in a later Prisma release).
• Verify with a full server image build — it only runs on push to main / release tags, not on PRs (docker/build-push-action, docker/server/Dockerfile). Consider a one-off workflow_dispatch image build to validate before merging.