feat: Phase A backend — notes CRUD, detail endpoints, dashboard fix

- Create kanzlai.notes table (polymorphic FK with CHECK constraint,
  partial indexes, RLS)
- Add Note model, NoteService (ListByParent, Create, Update, Delete),
  and NoteHandler with endpoints: GET/POST /api/notes, PUT/DELETE /api/notes/{id}
- Add GET /api/deadlines/{deadlineID} detail endpoint
- Add GET /api/appointments/{id} detail endpoint
- Add GET /api/case-events/{id} detail endpoint (new CaseEventHandler)
- Fix dashboard query: add case_id to upcoming_deadlines SELECT,
  add id and case_id to recent_activity SELECT
- Register all new routes in router.go

backend/internal/models/note.go

@@ -0,0 +1,20 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Note struct {
+	ID            uuid.UUID  `db:"id" json:"id"`
+	TenantID      uuid.UUID  `db:"tenant_id" json:"tenant_id"`
+	CaseID        *uuid.UUID `db:"case_id" json:"case_id,omitempty"`
+	DeadlineID    *uuid.UUID `db:"deadline_id" json:"deadline_id,omitempty"`
+	AppointmentID *uuid.UUID `db:"appointment_id" json:"appointment_id,omitempty"`
+	CaseEventID   *uuid.UUID `db:"case_event_id" json:"case_event_id,omitempty"`
+	Content       string     `db:"content" json:"content"`
+	CreatedBy     *uuid.UUID `db:"created_by" json:"created_by,omitempty"`
+	CreatedAt     time.Time  `db:"created_at" json:"created_at"`
+	UpdatedAt     time.Time  `db:"updated_at" json:"updated_at"`
+}