feat: Sprint 4 — scenario planner, report builder, comments, dashboard widgets

What-If Scenario Planner (G5):
- New /projects/[id]/scenario page with side-by-side baseline vs scenario
- simulate mutation: pure cost/hours/headcount/utilization computation
- apply mutation: creates real PROPOSED assignments from scenario
- Impact cards: cost delta, hours delta, headcount, skill coverage %
- Per-resource utilization impact table with over-allocation warnings
- "What-If" button added to project detail page

Custom Report Builder (G7):
- New /reports/builder page with full config panel
- Entity selector (resource/project/assignment), column picker, filter builder
- Dynamic Prisma query with eq/neq/gt/lt/contains/in operators
- Sortable results table with pagination (50/page)
- CSV export via exportReport mutation
- Sidebar nav link under Analytics

Collaboration Layer (G8):
- Comment model in Prisma (entityType/entityId, replies, @mentions, resolved)
- comment router: list, count, create, resolve, delete
- @mention parsing with notification creation + SSE delivery
- CommentInput with @mention autocomplete (arrow nav, Enter/Tab confirm)
- CommentThread with avatar, timestamp, reply, resolve, delete
- Integrated as "Comments" tab in estimate workspace with count badge

Dashboard Widgets:
- BudgetForecastWidget: progress bars per project, burn rate, exhaustion date
- SkillGapWidget: supply vs demand per skill, shortage/surplus indicators
- ProjectHealthWidget: 3-dimension health circles + composite score
- 3 new application use-cases + dashboard router queries
- All registered in widget-registry with lazy imports

Co-Authored-By: claude-flow <ruv@ruv.net>

packages/db/prisma/schema.prisma

@@ -190,6 +190,7 @@ model User {
   tasksAssigned      Notification[]          @relation("taskAssignee")
   notificationsSent  Notification[]          @relation("notificationSender")
   broadcasts         NotificationBroadcast[] @relation("broadcastSender")
+  comments           Comment[]
 
   createdAt         DateTime  @default(now())
   updatedAt         DateTime  @updatedAt
@@ -1481,6 +1482,29 @@ model CalculationRule {
   @@map("calculation_rules")
 }
 
+// ─── Comment ─────────────────────────────────────────────────────────────────
+
+model Comment {
+  id            String    @id @default(cuid())
+  entityType    String    // "estimate", "estimate_version", "scope_item", "demand_line"
+  entityId      String
+  parentId      String?   // for replies
+  authorId      String
+  body          String    @db.Text
+  mentions      String[]  // user IDs mentioned
+  resolved      Boolean   @default(false)
+  createdAt     DateTime  @default(now())
+  updatedAt     DateTime  @updatedAt
+
+  author        User      @relation(fields: [authorId], references: [id])
+  parent        Comment?  @relation("CommentReplies", fields: [parentId], references: [id])
+  replies       Comment[] @relation("CommentReplies")
+
+  @@index([entityType, entityId])
+  @@index([authorId])
+  @@map("comments")
+}
+
 // ─── Audit Log ────────────────────────────────────────────────────────────────
 
 model AuditLog {