feat(backend): ports CRUD endpoints for slice 7

New store methods on internal/db/ports.go:
- CreatePort / GetPort / UpdatePort / DeletePort (all project-scoped)
- ListPortsForDevice for the inspector's per-device list

New handlers (internal/server/ports.go):
- GET    /api/projects/:pid/devices/:id/ports
- POST   /api/projects/:pid/devices/:id/ports  ← {type_id, label?, x_offset, y_offset}
- PATCH  /api/projects/:pid/ports/:id           ← partial
- DELETE /api/projects/:pid/ports/:id          (cables ref → ON DELETE SET NULL)

Lets slice 7's +Port tool add/remove instance ports without going
through the type-seeded auto-creation path from slice 4.

internal/server/ports.go

@@ -0,0 +1,114 @@
+package server
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"mgit.msbls.de/m/mcables/internal/db"
+)
+
+type portCreate struct {
+	TypeID  int64   `json:"type_id"`
+	Label   string  `json:"label,omitempty"`
+	XOffset float64 `json:"x_offset"`
+	YOffset float64 `json:"y_offset"`
+}
+
+type portPatch struct {
+	TypeID  *int64   `json:"type_id,omitempty"`
+	Label   *string  `json:"label,omitempty"`
+	XOffset *float64 `json:"x_offset,omitempty"`
+	YOffset *float64 `json:"y_offset,omitempty"`
+}
+
+func (h *handlers) listPortsForDevice(w http.ResponseWriter, r *http.Request) {
+	pid, ok := parseInt64Path(r, "pid")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "pid must be a positive integer")
+		return
+	}
+	id, ok := parseInt64Path(r, "id")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "id must be a positive integer")
+		return
+	}
+	ps, err := h.store.ListPortsForDevice(pid, id)
+	if err != nil {
+		writeError(w, err, nil)
+		return
+	}
+	writeJSON(w, http.StatusOK, ps)
+}
+
+func (h *handlers) createPort(w http.ResponseWriter, r *http.Request) {
+	pid, ok := parseInt64Path(r, "pid")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "pid must be a positive integer")
+		return
+	}
+	id, ok := parseInt64Path(r, "id")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "id must be a positive integer")
+		return
+	}
+	var body portCreate
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, errors.Join(db.ErrInvalidInput, err), nil)
+		return
+	}
+	p, err := h.store.CreatePort(pid, id, db.PortCreate{
+		TypeID: body.TypeID, Label: body.Label,
+		XOffset: body.XOffset, YOffset: body.YOffset,
+	})
+	if err != nil {
+		writeError(w, err, nil)
+		return
+	}
+	writeJSON(w, http.StatusCreated, p)
+}
+
+func (h *handlers) patchPort(w http.ResponseWriter, r *http.Request) {
+	pid, ok := parseInt64Path(r, "pid")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "pid must be a positive integer")
+		return
+	}
+	id, ok := parseInt64Path(r, "id")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "id must be a positive integer")
+		return
+	}
+	var body portPatch
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, errors.Join(db.ErrInvalidInput, err), nil)
+		return
+	}
+	p, err := h.store.UpdatePort(pid, id, db.PortUpdate{
+		TypeID: body.TypeID, Label: body.Label,
+		XOffset: body.XOffset, YOffset: body.YOffset,
+	})
+	if err != nil {
+		writeError(w, err, nil)
+		return
+	}
+	writeJSON(w, http.StatusOK, p)
+}
+
+func (h *handlers) deletePort(w http.ResponseWriter, r *http.Request) {
+	pid, ok := parseInt64Path(r, "pid")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "pid must be a positive integer")
+		return
+	}
+	id, ok := parseInt64Path(r, "id")
+	if !ok {
+		writeError(w, db.ErrInvalidInput, "id must be a positive integer")
+		return
+	}
+	if err := h.store.DeletePort(pid, id); err != nil {
+		writeError(w, err, nil)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}

internal/server/server.go

@@ -51,6 +51,12 @@ func New(store *db.Store, frontend fs.FS) http.Handler {
 	mux.HandleFunc("PATCH /api/projects/{pid}/io-markers/{id}", h.patchIOMarker)
 	mux.HandleFunc("DELETE /api/projects/{pid}/io-markers/{id}", h.deleteIOMarker)
 
+	// Ports — slice 7 lets m add/edit/remove instance ports on a device.
+	mux.HandleFunc("GET /api/projects/{pid}/devices/{id}/ports", h.listPortsForDevice)
+	mux.HandleFunc("POST /api/projects/{pid}/devices/{id}/ports", h.createPort)
+	mux.HandleFunc("PATCH /api/projects/{pid}/ports/{id}", h.patchPort)
+	mux.HandleFunc("DELETE /api/projects/{pid}/ports/{id}", h.deletePort)
+
 	// Device-type catalog. Built-ins are read-only; project-custom rows
 	// support full CRUD scoped to the project.
 	mux.HandleFunc("GET /api/device-types", h.listBuiltInDeviceTypes)