Switch backend to Node 25 with V8 pointer compression (node-caged)

[rbessin]

Changes

• src/backend/Dockerfile — switched from node:20 to platformatic/node-caged:25-slim. Added npm install -g yarn since the node-caged slim image doesn't include yarn by default.
• Dockerfile (root) — switched builder stage from node:20 to node:25, production stage from node:20-slim to platformatic/node-caged:25-slim.
• devContainerization/Dockerfile.backend.dev — switched from node:20 to node:25 for dev/prod parity.

Notes

• The seed command fails with a pre-existing materialId column error in reimbursement-requests.seed.ts; this exists on develop too and is unrelated to these changes
• Pointer compression requires Node 25; official Node builds don't include it yet, which is why the node-caged image is needed

Test Cases

• Dockerfile (root / CI): built successfully, all 26 migrations applied, full browser testing completed — login, projects, work packages, finance, and reimbursement all functional
• src/backend/Dockerfile (production / yarn containerize): built successfully, all 26 migrations applied, limited browser testing due to seed failure preventing organization data from being created
• devContainerization/Dockerfile.backend.dev (dev): build only, not runtime-tested
• No NAN dependency conflicts (npm ls nan returns empty)
• Note: local memory numbers (~280-325MB) are not directly comparable to production peak; real savings would be visible on EC2 under actual traffic

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Closes # NONE

[rbessin]

I More testing should be done but I was struggling to complete it due to the seeding issue (materialId column error in the reimbursement seed). The dev environment (yarn docker:dev) was only build-tested, not runtime-tested, and yarn containerize browser testing was limited since the app couldn't load without organization data. If the seed issue is fixed, full runtime testing across all three environments would be straightforward.