feat: add HEIC/HEIF format support for input and output

Add bidirectional HEIC support using system libheif CLI tools (heif-enc/heif-dec)
for HEVC encoding/decoding, since Sharp's bundled libheif only supports AV1.

- HEIC input: all tools now accept iPhone HEIC photos via heif-dec pre-processing
- HEIC output: convert tool produces true HEIC (HEVC) via heif-enc
- Docker: adds libheif-examples package for heif-enc/heif-dec CLI tools
- Tests: full conversion matrix (7x7), unit tests, and Playwright e2e tests
- Docs: updated OpenAPI spec, image-engine docs, getting-started, llms-full.txt

apps/api/src/openapi.yaml

@@ -266,7 +266,7 @@ paths:
                   type: string
                   description: |
                     JSON string with options:
-                    - `format` (string, required) — One of: jpg, png, webp, avif, tiff, gif
+                    - `format` (string, required) — One of: jpg, png, webp, avif, tiff, gif, heic
                     - `quality` (number 1-100, optional) — Output quality
       responses:
         "200":