Optimize CI build pipeline (~15 min → ~3-5 min) (#59)

## Summary

- **Dockerfile layer caching**: `development-dependencies-env` now copies only `package.json`/`package-lock.json` before `npm ci` (was `COPY . /app`), so the npm install layer is cached on every code-only commit instead of rebuilt from scratch
- **Skip QEMU**: `platforms: linux/amd64` added to `build-push-action`, cutting `setup-buildx-action` from ~5 min to ~30 sec
- **npm cache in CI jobs**: Manual `actions/cache@v3` blocks (copy-pasted 3×) replaced with `actions/setup-node@v4` + `cache: 'npm'`, which handles path/key/restore automatically and pins Node 20 explicitly
- **Harden `.npmrc`**: Switched from `COPY .npmrc` to `--mount=type=secret,id=npmrc` in both `npm ci` stages — the file is available during install but never written into a Docker layer, so it cannot leak through the registry build cache regardless of future contents

## Expected timing

| Step | Before | After (code change) |
|---|---|---|
| Setup buildx | ~5 min | ~30 sec |
| Build + push | ~5 min | ~1-2 min |
| Deploy (docker pull) | ~5 min | ~1-2 min |
| **Total** | **~15 min** | **~3-5 min** |

## Test plan

- [ ] Push a code-only commit to main — confirm `setup-buildx-action` logs ~30s (no QEMU), Docker build shows `CACHED` for npm install layers, build+push completes in ~1-2 min
- [ ] Check deploy job — `docker compose pull` should show most layers as `Already exists`
- [ ] Push a commit that changes `package.json` — confirm npm layer correctly re-runs (not cached)
- [ ] Confirm deployed app is functional

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Chris Parsons <chrisparsons1127@gmail.com>
Reviewed-on: #59

.forgejo/workflows/deploy.yml

@@ -100,9 +100,12 @@ jobs:
               with:
                   context: .
                   push: true
+                  platforms: linux/amd64
                   tags: ${{ vars.CONTAINER_REGISTRY }}/brackt:latest
                   cache-from: type=registry,ref=${{ vars.CONTAINER_REGISTRY }}/brackt:buildcache
                   cache-to: type=registry,ref=${{ vars.CONTAINER_REGISTRY }}/brackt:buildcache,mode=max
+                  secret-files: |
+                      npmrc=./.npmrc
 
     deploy:
         name: 🚀 Deploy

Dockerfile

@@ -1,12 +1,12 @@
 FROM node:20-alpine AS development-dependencies-env
-COPY . /app
+COPY package.json package-lock.json /app/
 WORKDIR /app
-RUN npm ci
+RUN --mount=type=secret,id=npmrc,target=/app/.npmrc npm ci
 
 FROM node:20-alpine AS production-dependencies-env
-COPY ./package.json package-lock.json .npmrc /app/
+COPY ./package.json package-lock.json /app/
 WORKDIR /app
-RUN npm ci --omit=dev
+RUN --mount=type=secret,id=npmrc,target=/app/.npmrc npm ci --omit=dev
 
 FROM node:20-alpine AS build-env
 COPY . /app/