Delete dead test helpers and rename vestigial test paths

- Remove seed_helpers.go (SeedTestMessage, SeedTestMessages,
  SeedTestCursor — all unused, channel/message features gutted)
- Rename channel- prefixed test paths to test- in storage tests

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

server/database/seed_helpers.go

@@ -1,62 +0,0 @@
-package database
-
-import (
-	"strings"
-	"testing"
-)
-
-// ── Additional Test Seed Helpers (v0.15.0) ──
-
-// SeedTestMessage creates a single message in a channel and returns the message ID.
-func SeedTestMessage(t *testing.T, channelID, parentID, role, content string) string {
-	t.Helper()
-	var id string
-	var parentPtr *string
-	if parentID != "" {
-		parentPtr = &parentID
-	}
-	err := DB.QueryRow(`
-		INSERT INTO messages (channel_id, parent_id, role, content, sibling_index)
-		VALUES ($1, $2, $3, $4, 0)
-		RETURNING id
-	`, channelID, parentPtr, role, content).Scan(&id)
-	if err != nil {
-		t.Fatalf("SeedTestMessage: %v", err)
-	}
-	return id
-}
-
-// SeedTestMessages creates a linear chain of alternating user/assistant messages.
-// Returns all message IDs in order. The first message has no parent.
-func SeedTestMessages(t *testing.T, channelID string, count int, contentSize int) []string {
-	t.Helper()
-	content := strings.Repeat("x", contentSize)
-	ids := make([]string, 0, count)
-
-	parentID := ""
-	for i := 0; i < count; i++ {
-		role := "user"
-		if i%2 == 1 {
-			role = "assistant"
-		}
-		id := SeedTestMessage(t, channelID, parentID, role, content)
-		ids = append(ids, id)
-		parentID = id
-	}
-
-	return ids
-}
-
-// SeedTestCursor sets the active leaf for a user in a channel.
-func SeedTestCursor(t *testing.T, channelID, userID, leafID string) {
-	t.Helper()
-	_, err := DB.Exec(`
-		INSERT INTO channel_cursors (channel_id, user_id, active_leaf_id)
-		VALUES ($1, $2, $3)
-		ON CONFLICT (channel_id, user_id)
-		DO UPDATE SET active_leaf_id = $3, updated_at = NOW()
-	`, channelID, userID, leafID)
-	if err != nil {
-		t.Fatalf("SeedTestCursor: %v", err)
-	}
-}