feat(api): add comprehensive ampcode management endpoints

Add new REST API endpoints under /v0/management/ampcode for managing
ampcode configuration including upstream URL, API key, localhost
restriction, model mappings, and force model mappings settings.

- Move force-model-mappings from config_basic to config_lists
- Add GET/PUT/PATCH/DELETE endpoints for all ampcode settings
- Support model mapping CRUD with upsert (PATCH) capability
- Add comprehensive test coverage for all ampcode endpoints

internal/api/handlers/management/config_basic.go

@@ -241,11 +241,3 @@ func (h *Handler) DeleteProxyURL(c *gin.Context) {
 	h.cfg.ProxyURL = ""
 	h.persist(c)
 }
-
-// Force Model Mappings (for Amp CLI)
-func (h *Handler) GetForceModelMappings(c *gin.Context) {
-	c.JSON(200, gin.H{"force-model-mappings": h.cfg.AmpCode.ForceModelMappings})
-}
-func (h *Handler) PutForceModelMappings(c *gin.Context) {
-	h.updateBoolField(c, func(v bool) { h.cfg.AmpCode.ForceModelMappings = v })
-}