feat: bounty board, task lifecycle, AI work performer flow, notifications (phase 8)

- whitelist HTML sanitizer (stdlib tokenizer) with XSS vector tests
- developer board: pool-scoped visibility, customer/search/minBounty/sort
  filters, stale-task age badges, competing-claims visibility setting,
  saved filters in profile extras
- claims: request/withdraw (developer), approve/decline (consultant) with
  notifications to winners and losers; unassign/abandon back to board
- work tracking: start, sanitized comments with @mention notifications,
  time logging, submit for review
- review queue + review with per-AC checklist stored on the timeline;
  approve writes the immutable bountyAwards row (human assignees only)
- assign-to-AI: §5.2 job submission, HMAC-verified callback endpoint,
  idempotent by jobId, artifacts downloaded into GridFS, failure path
  keeps the task assigned with timeline + notification
- notifications API + bell with unread badge, dropdown, page, WS toasts
- pages: bounty board, my-tasks kanban, task detail (role-driven actions,
  review dialog, AI dialog), review queue, developer pool

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/store/awards.go

@@ -0,0 +1,31 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"bountyboard/internal/ulid"
+)
+
+// BountyAward is the immutable ledger row (§4.5) — never updated, never
+// recomputed from mutable tasks.
+type BountyAward struct {
+	ID           string    `bson:"_id" json:"id"`
+	TaskID       string    `bson:"taskId" json:"taskId"`
+	DeveloperID  string    `bson:"developerId" json:"developerId"`
+	ConsultantID string    `bson:"consultantId" json:"consultantId"`
+	CustomerID   string    `bson:"customerId" json:"customerId"`
+	Amount       float64   `bson:"amount" json:"amount"`
+	Coefficient  float64   `bson:"coefficient" json:"coefficient"`
+	AwardedAt    time.Time `bson:"awardedAt" json:"awardedAt"`
+}
+
+func (s *Store) InsertAward(ctx context.Context, a *BountyAward) error {
+	a.ID = ulid.New()
+	a.AwardedAt = time.Now().UTC()
+	if _, err := s.DB.Collection("bountyAwards").InsertOne(ctx, a); err != nil {
+		return fmt.Errorf("insert award: %w", err)
+	}
+	return nil
+}