Perf: Optimiser builds backend avec image de base (90-95% plus rapide)

Architecture en 2 images:
- Image base (audio-classifier-base): deps système + Python (~15min, 1x/semaine)
- Image app (audio-classifier-backend): code uniquement (~30s-2min, chaque commit)

Fichiers ajoutés:
- backend/Dockerfile.base: Image de base avec toutes les dépendances
- .gitea/workflows/docker-base.yml: CI pour build de l'image de base
- backend/DOCKER_BUILD.md: Documentation complète

Fichiers modifiés:
- backend/Dockerfile: Utilise l'image de base (FROM audio-classifier-base)
- .gitea/workflows/docker.yml: Passe BASE_IMAGE en build-arg

Gains de performance:
- Build normal: 15-25min → 30s-2min (90-95% plus rapide)
- Trigger auto du build base: quand requirements.txt change
- Trigger manuel: via interface Gitea Actions

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

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/docker-base.yml

@@ -0,0 +1,61 @@
+name: Build Base Docker Image
+
+# Build base image only when requirements.txt changes or manually triggered
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - 'backend/requirements.txt'
+      - 'backend/Dockerfile.base'
+  workflow_dispatch:  # Allow manual trigger
+
+env:
+  REGISTRY: git.benoitsz.com
+  IMAGE_BASE: audio-classifier-base
+
+jobs:
+  build-base:
+    name: Build Base Image
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Gitea Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ gitea.repository_owner }}/${{ env.IMAGE_BASE }}
+          tags: |
+            type=raw,value=latest
+            type=sha,prefix=sha-,format=short
+
+      - name: Build and push base image
+        uses: docker/build-push-action@v5
+        with:
+          context: ./backend
+          file: ./backend/Dockerfile.base
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ gitea.repository_owner }}/${{ env.IMAGE_BASE }}:buildcache
+          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ gitea.repository_owner }}/${{ env.IMAGE_BASE }}:buildcache,mode=max
+          platforms: linux/amd64
+
+      - name: Image built successfully
+        run: |
+          echo "✅ Base image built and pushed successfully"
+          echo "📦 Image: ${{ env.REGISTRY }}/${{ gitea.repository_owner }}/${{ env.IMAGE_BASE }}:latest"
+          echo "⏱️  This image will be used by the main backend builds to speed up CI/CD"