feat(webp): add WebP streaming and snapshot APIs

- implement WebP streaming with multipart support
- add snapshot endpoint for WebP format with quality options
- introduce WebP encoding using pure Go library without FFmpeg
- update documentation and links for new WebP features

www/schema.json

@@ -151,6 +151,7 @@
               "mp4",
               "hls",
               "mjpeg",
+              "webp",
               "hass",
               "homekit",
               "onvif",
@@ -427,6 +428,9 @@
         "mjpeg": {
           "$ref": "#/definitions/log_level"
         },
+        "webp": {
+          "$ref": "#/definitions/log_level"
+        },
         "mp4": {
           "$ref": "#/definitions/log_level"
         },