feat(notifications): replace hardcoded inbox notifications with subscriber-driven model

Replace inbox_listeners.go with a subscriber-driven notification system:

- Add issue_subscriber table with auto-subscribe on create/assign/comment
- New subscriber_listeners.go: maintains subscriber data on domain events
- New notification_listeners.go: notifySubscribers (fanout to all subscribers
  minus actor) and notifyDirect (targeted, punches through unsubscribe)
- Subscriber API: list/subscribe/unsubscribe endpoints
- Frontend: subscribers section in issue detail sidebar with real-time sync
- Frontend: inbox notification grouping by (issue_id, type, actor_id)
- Remove createInboxForIssueCreator from task.go (unified through event bus)
- 21 new Go tests, all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/generated/subscriber.sql.go

@@ -0,0 +1,103 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.30.0
+// source: subscriber.sql
+
+package db
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const addIssueSubscriber = `-- name: AddIssueSubscriber :exec
+INSERT INTO issue_subscriber (issue_id, user_type, user_id, reason)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (issue_id, user_type, user_id) DO NOTHING
+`
+
+type AddIssueSubscriberParams struct {
+	IssueID  pgtype.UUID `json:"issue_id"`
+	UserType string      `json:"user_type"`
+	UserID   pgtype.UUID `json:"user_id"`
+	Reason   string      `json:"reason"`
+}
+
+func (q *Queries) AddIssueSubscriber(ctx context.Context, arg AddIssueSubscriberParams) error {
+	_, err := q.db.Exec(ctx, addIssueSubscriber,
+		arg.IssueID,
+		arg.UserType,
+		arg.UserID,
+		arg.Reason,
+	)
+	return err
+}
+
+const isIssueSubscriber = `-- name: IsIssueSubscriber :one
+SELECT EXISTS(
+    SELECT 1 FROM issue_subscriber
+    WHERE issue_id = $1 AND user_type = $2 AND user_id = $3
+) AS subscribed
+`
+
+type IsIssueSubscriberParams struct {
+	IssueID  pgtype.UUID `json:"issue_id"`
+	UserType string      `json:"user_type"`
+	UserID   pgtype.UUID `json:"user_id"`
+}
+
+func (q *Queries) IsIssueSubscriber(ctx context.Context, arg IsIssueSubscriberParams) (bool, error) {
+	row := q.db.QueryRow(ctx, isIssueSubscriber, arg.IssueID, arg.UserType, arg.UserID)
+	var subscribed bool
+	err := row.Scan(&subscribed)
+	return subscribed, err
+}
+
+const listIssueSubscribers = `-- name: ListIssueSubscribers :many
+SELECT issue_id, user_type, user_id, reason, created_at FROM issue_subscriber
+WHERE issue_id = $1
+ORDER BY created_at
+`
+
+func (q *Queries) ListIssueSubscribers(ctx context.Context, issueID pgtype.UUID) ([]IssueSubscriber, error) {
+	rows, err := q.db.Query(ctx, listIssueSubscribers, issueID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	items := []IssueSubscriber{}
+	for rows.Next() {
+		var i IssueSubscriber
+		if err := rows.Scan(
+			&i.IssueID,
+			&i.UserType,
+			&i.UserID,
+			&i.Reason,
+			&i.CreatedAt,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const removeIssueSubscriber = `-- name: RemoveIssueSubscriber :exec
+DELETE FROM issue_subscriber
+WHERE issue_id = $1 AND user_type = $2 AND user_id = $3
+`
+
+type RemoveIssueSubscriberParams struct {
+	IssueID  pgtype.UUID `json:"issue_id"`
+	UserType string      `json:"user_type"`
+	UserID   pgtype.UUID `json:"user_id"`
+}
+
+func (q *Queries) RemoveIssueSubscriber(ctx context.Context, arg RemoveIssueSubscriberParams) error {
+	_, err := q.db.Exec(ctx, removeIssueSubscriber, arg.IssueID, arg.UserType, arg.UserID)
+	return err
+}