feat: add /render/mp4 endpoint with FFmpeg encoding

- Add MP4Request type with fps param
- Render frames in parallel, encode via ffmpeg
- Add ffmpeg to Docker image

internal/api/handlers.go

@@ -113,6 +113,43 @@ func (h *Handlers) HandleGIF(w http.ResponseWriter, r *http.Request) {
 	w.Write(gifBytes)
 }
 
+// HandleMP4 handles POST /render/mp4
+func (h *Handlers) HandleMP4(w http.ResponseWriter, r *http.Request) {
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "failed to read request body")
+		return
+	}
+	defer r.Body.Close()
+
+	var req MP4Request
+	if err := json.Unmarshal(body, &req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid JSON: "+err.Error())
+		return
+	}
+	req.ApplyDefaults()
+
+	if req.Character == nil {
+		writeError(w, http.StatusBadRequest, "character field is required")
+		return
+	}
+
+	result, err := h.svc.MergeFromJSON(req.Character)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "merge failed: "+err.Error())
+		return
+	}
+
+	mp4Bytes, err := render.RenderMP4(result.GLBBytes, result.Atlas, req.Background, req.Frames, req.Width, req.Height, req.FPS)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "render failed: "+err.Error())
+		return
+	}
+
+	w.Header().Set("Content-Type", "video/mp4")
+	w.Write(mp4Bytes)
+}
+
 // HandleHealth handles GET /health
 func (h *Handlers) HandleHealth(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "application/json")

internal/api/openapi.go

@@ -150,6 +150,43 @@ const OpenAPISpec = `{
           }
         }
       }
+    },
+    "/render/mp4": {
+      "post": {
+        "summary": "Render character as MP4 video",
+        "description": "Renders a character as an MP4 video rotating 360 degrees.",
+        "operationId": "renderMP4",
+        "tags": ["Render"],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/MP4Request"
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "MP4 video",
+            "content": {
+              "video/mp4": {
+                "schema": {
+                  "type": "string",
+                  "format": "binary"
+                }
+              }
+            }
+          },
+          "400": {
+            "$ref": "#/components/responses/BadRequest"
+          },
+          "500": {
+            "$ref": "#/components/responses/InternalError"
+          }
+        }
+      }
     }
   },
   "components": {
@@ -204,6 +241,18 @@ const OpenAPISpec = `{
           "dithering": {"type": "boolean", "default": true, "description": "Enable Floyd-Steinberg dithering (disable for faster rendering)"}
         }
       },
+      "MP4Request": {
+        "type": "object",
+        "required": ["character"],
+        "properties": {
+          "character": {"$ref": "#/components/schemas/CharacterConfig"},
+          "background": {"type": "string", "default": "#FFFFFF", "description": "Hex color background"},
+          "frames": {"type": "integer", "default": 36, "description": "Number of frames (36 = 10° per frame)"},
+          "width": {"type": "integer", "default": 512, "description": "Video width in pixels"},
+          "height": {"type": "integer", "default": 512, "description": "Video height in pixels"},
+          "fps": {"type": "integer", "default": 12, "description": "Frames per second"}
+        }
+      },
       "ErrorResponse": {
         "type": "object",
         "properties": {

internal/api/server.go

@@ -29,6 +29,7 @@ func NewServer(svc *service.MergeService) http.Handler {
 	r.Post("/render/glb", h.HandleGLB)
 	r.Post("/render/png", h.HandlePNG)
 	r.Post("/render/gif", h.HandleGIF)
+	r.Post("/render/mp4", h.HandleMP4)
 
 	return r
 }

internal/api/types.go

@@ -22,6 +22,16 @@ type GIFRequest struct {
 	Dithering  *bool           `json:"dithering"`  // Floyd-Steinberg dithering, default true
 }
 
+// MP4Request represents a request to render a character as MP4 video
+type MP4Request struct {
+	Character  json.RawMessage `json:"character"`
+	Background string          `json:"background"` // hex color "#RRGGBB", default "#FFFFFF"
+	Frames     int             `json:"frames"`     // default 36 (10° per frame)
+	Width      int             `json:"width"`      // default 512
+	Height     int             `json:"height"`     // default 512
+	FPS        int             `json:"fps"`        // frames per second, default 12
+}
+
 // ErrorResponse represents an error returned by the API
 type ErrorResponse struct {
 	Error string `json:"error"`
@@ -62,3 +72,22 @@ func (r *GIFRequest) ApplyDefaults() {
 		r.Dithering = &defaultDithering
 	}
 }
+
+// ApplyDefaults fills in default values for MP4Request
+func (r *MP4Request) ApplyDefaults() {
+	if r.Width == 0 {
+		r.Width = 512
+	}
+	if r.Height == 0 {
+		r.Height = 512
+	}
+	if r.Frames == 0 {
+		r.Frames = 36
+	}
+	if r.FPS == 0 {
+		r.FPS = 12
+	}
+	if r.Background == "" {
+		r.Background = "#FFFFFF"
+	}
+}