feat: lightweight Docker image without AI/ML tools (:lite tag)

Closes #1

apps/docs/guide/docker-tags.md

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+# Docker Image Tags
+
+Stirling Image ships two Docker image variants to fit different use cases.
+
+## Full (default)
+
+```bash
+docker pull stirlingimage/stirling-image:latest
+```
+
+Includes all tools: image processing, AI-powered background removal, upscaling, face blurring, object erasing, and OCR. Size is ~11 GB due to bundled ML models.
+
+## Lite
+
+```bash
+docker pull stirlingimage/stirling-image:lite
+```
+
+Includes all image processing tools (resize, crop, rotate, convert, compress, watermark, collage, and 20+ more) but excludes AI/ML tools. Size is ~1-2 GB.
+
+Use this if you:
+- Only need standard image processing (no AI features)
+- Are running on constrained hardware (Raspberry Pi, small VPS)
+- Want faster pulls and smaller disk footprint
+
+### Tools excluded from lite
+
+| Tool | What it does |
+|------|-------------|
+| Remove Background | AI-powered background removal |
+| Upscale | AI super-resolution upscaling |
+| Blur Faces | AI face detection and blurring |
+| Erase Object | AI inpainting to remove objects |
+| OCR | Optical character recognition |
+
+All other tools (27+) work identically in both variants.
+
+## Docker Compose
+
+### Full
+
+```yaml
+services:
+  stirling-image:
+    image: stirlingimage/stirling-image:latest
+    ports:
+      - "1349:1349"
+    volumes:
+      - stirling-data:/data
+      - stirling-workspace:/tmp/workspace
+
+volumes:
+  stirling-data:
+  stirling-workspace:
+```
+
+### Lite
+
+```yaml
+services:
+  stirling-image:
+    image: stirlingimage/stirling-image:lite
+    ports:
+      - "1349:1349"
+    volumes:
+      - stirling-data:/data
+      - stirling-workspace:/tmp/workspace
+
+volumes:
+  stirling-data:
+  stirling-workspace:
+```
+
+## Switching from lite to full
+
+To upgrade from lite to full and unlock AI tools:
+
+1. Stop your container
+2. Pull the full image: `docker pull stirlingimage/stirling-image:latest`
+3. Update your compose file or run command to use `:latest` instead of `:lite`
+4. Start the container
+
+Your data and settings are preserved in the volumes.
+
+## Version pinning
+
+Both variants support semver tags for pinning:
+
+| Tag | Description |
+|-----|------------|
+| `latest` | Latest full release |
+| `lite` | Latest lite release |
+| `1.6.0` | Exact full version |
+| `1.6.0-lite` | Exact lite version |
+| `1.6` | Latest patch in 1.6.x (full) |
+| `1.6-lite` | Latest patch in 1.6.x (lite) |