test(server): fixture collisions fixed, stale contract asserts re-anchored, gofmt — suite now fully green

Closes the remaining integration-suite failures (handoff open bug 3),
line by line:

- Fixture family (the upsert equivalence): tags_filter_test
  ("Science Fiction" found 1 of 3; "Fantasy" 404'd on zero results),
  media_item_isbn_test (normalization read back the first case's ISBN
  everywhere; edge cases saw a nil ISBN; pagination collapsed 5
  creates onto 1 row), search_unified_test (its 3 books were actually
  1 row + two no-op creates) — all POSTed a constant file_path into
  one library while CreateMediaItem upserts ON CONFLICT
  (library_id, file_path). Every fixture now uses unique per-item
  paths, the suite's universal convention elsewhere. TestTagsFilter
  confirmed same-family: counts came right with no second bug.
- New TestMediaItemCreateDuplicatePath pins the honest-create
  contract from da13898: 201 for the first create; 200 + duplicate
  flag + the EXISTING row (never a 201 lie), second payload not
  overwritten.
- library_browse_test: the admin subtest browsed the container's
  /tmp and required it non-empty — env-coupled (empty /tmp → "was
  []"). Now browses a self-created t.TempDir with two subdirs and a
  decoy file, asserting ONLY directories are listed.
- series_integration_test: three stale assertions re-anchored to the
  shipped contracts, each with a ground-truth comment: /dashboard/
  sections key identity on the collection NAME (BuildSections ID =
  CollectionName; the web/app reorder + hidden prefs persist those
  strings) — the kebab query_type is the storage key and is not
  exposed; /api/series/books is name-ONLY (the app client and the
  web SSR page both send name only; library_id is ignored, never
  validated); /api/series is optional-scope BY DESIGN (GetDistinct-
  Series: narg IS NULL OR library_id = narg aggregates across
  libraries when omitted). The former 400-on-missing-library_id
  expectations asserted validation that never shipped.
- gofmt rider on three untouched-in-this-arc test files
  (dashboard_integration, search, test_helpers).

Full suite with -parallel=1: 217 groups pass, zero failures.

cmd/server/tests/search_test.go

@@ -384,12 +384,12 @@ func TestCollectionSearchLibraryFilter(t *testing.T) {
 	book2ID := createTestMediaItemIDInLibrary(t, client, setup, lib2Resp["id"].(string), "Harry Potter 2")
 
 	tests := []struct {
-		name          string
-		query         string
-		libraryID     string
-		expectedCount int
+		name           string
+		query          string
+		libraryID      string
+		expectedCount  int
 		expectedStatus int
-		shouldContain string // Comma-separated list of book IDs to check
+		shouldContain  string // Comma-separated list of book IDs to check
 	}{
 		{
 			name:          "no filter - both test books present",