perf(ci): cache Next.js build artifacts with Blacksmith cache

Add useblacksmith/cache@v5 to persist web/.next/cache across CI runs,
eliminating redundant Webpack recompilation of unchanged pages. Applied
to ci.yml build job and pipeline.yml dev-publish/prod-release jobs.

.github/workflows/ci.yml

@@ -130,6 +130,15 @@ jobs:
       - name: Install web host dependencies
         run: npm --prefix web ci
 
+      - name: Cache Next.js build
+        uses: useblacksmith/cache@v5
+        with:
+          path: web/.next/cache
+          key: nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-${{ hashFiles('web/app/**', 'web/components/**', 'web/lib/**', 'web/hooks/**') }}
+          restore-keys: |
+            nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-
+            nextjs-${{ runner.os }}-
+
       - name: Build
         run: npm run build
 

.github/workflows/pipeline.yml

@@ -24,6 +24,10 @@ jobs:
       credentials:
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
+      env:
+        BLACKSMITH_CACHE_TOKEN: ${{ env.BLACKSMITH_CACHE_TOKEN }}
+        BLACKSMITH_CACHE_URL: ${{ env.BLACKSMITH_CACHE_URL }}
+        GITHUB_REPO_NAME: ${{ github.repository }}
     outputs:
       dev-version: ${{ steps.stamp.outputs.version }}
     steps:
@@ -41,6 +45,15 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Cache Next.js build
+        uses: useblacksmith/cache@v5
+        with:
+          path: web/.next/cache
+          key: nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-${{ hashFiles('web/app/**', 'web/components/**', 'web/lib/**', 'web/hooks/**') }}
+          restore-keys: |
+            nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-
+            nextjs-${{ runner.os }}-
+
       - name: Build
         run: npm run build
 
@@ -153,6 +166,15 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Cache Next.js build
+        uses: useblacksmith/cache@v5
+        with:
+          path: web/.next/cache
+          key: nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-${{ hashFiles('web/app/**', 'web/components/**', 'web/lib/**', 'web/hooks/**') }}
+          restore-keys: |
+            nextjs-${{ runner.os }}-${{ hashFiles('web/package-lock.json') }}-
+            nextjs-${{ runner.os }}-
+
       - name: Run live LLM tests (optional)
         continue-on-error: true
         run: npm run test:live