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/media_item_isbn_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 
@@ -151,11 +152,15 @@ func TestMediaItemISBNNormalization(t *testing.T) {
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			// Unique file_path per case: CreateMediaItem upserts ON
+			// CONFLICT (library_id, file_path), so a constant path would
+			// return the FIRST row's ISBN for every later case.
+			slug := strings.ReplaceAll(strings.ToLower(tc.name), " ", "-")
 			payload := map[string]interface{}{
 				"title":      fmt.Sprintf("Test Book %s", tc.name),
 				"isbn":       tc.input,
 				"library_id": libID,
-				"file_path":  "/test/path.epub",
+				"file_path":  "/test/" + slug + ".epub",
 				"file_size":  1024,
 				"mime_type":  "application/epub+zip",
 			}
@@ -203,7 +208,7 @@ func TestMediaItemISBNEdgeCases(t *testing.T) {
 		payload := map[string]interface{}{
 			"title":      "Book without ISBN",
 			"library_id": libID,
-			"file_path":  "/test/path.epub",
+			"file_path":  "/test/book-without-isbn.epub",
 			"file_size":  1024,
 			"mime_type":  "application/epub+zip",
 		}
@@ -228,7 +233,7 @@ func TestMediaItemISBNEdgeCases(t *testing.T) {
 			"title":      "Multi-Hyphen ISBN",
 			"isbn":       "978-0-306-40615-7",
 			"library_id": libID,
-			"file_path":  "/test/path.epub",
+			"file_path":  "/test/multi-hyphen-isbn.epub",
 			"file_size":  1024,
 			"mime_type":  "application/epub+zip",
 		}
@@ -258,7 +263,7 @@ func TestMediaItemISBNEdgeCases(t *testing.T) {
 			"title":      "Trailing Hyphen ISBN",
 			"isbn":       "978-0-596-00965-2",
 			"library_id": libID,
-			"file_path":  "/test/path.epub",
+			"file_path":  "/test/trailing-hyphen-isbn.epub",
 			"file_size":  1024,
 			"mime_type":  "application/epub+zip",
 		}
@@ -292,13 +297,15 @@ func TestMediaItemsPagination(t *testing.T) {
 	libID := createTestLibrary(t, setup.Server, token, "test-pagination-lib")
 	addFolderToLibrary(t, setup, libID, "/app/uploads")
 
-	// Create some test media items
+	// Create some test media items. Unique file_path per item: the create
+	// endpoint upserts ON CONFLICT (library_id, file_path), so a constant
+	// path would collapse all five creates onto one row.
 	for i := 1; i <= 5; i++ {
 		payload := map[string]interface{}{
 			"title":      fmt.Sprintf("Book %d", i),
 			"isbn":       fmt.Sprintf("978030640615%d", i),
 			"library_id": libID,
-			"file_path":  "/test/path.epub",
+			"file_path":  fmt.Sprintf("/test/book-%d.epub", i),
 			"file_size":  1024,
 			"mime_type":  "application/epub+zip",
 		}
@@ -408,6 +415,62 @@ func TestMediaItemsPagination(t *testing.T) {
 	})
 }
 
+// TestMediaItemCreateDuplicatePath pins the create endpoint's duplicate
+// behavior: a POST with a (library_id, file_path) that already exists
+// answers 200 OK with the EXISTING item and duplicate=true — never a
+// 201 with the old row pretending to be new, and nothing overwritten.
+func TestMediaItemCreateDuplicatePath(t *testing.T) {
+	setup := setupTestServer(t)
+
+	token := setup.Token
+	libID := createTestLibrary(t, setup.Server, token, "test-dup-create-lib")
+	addFolderToLibrary(t, setup, libID, "/app/uploads")
+
+	createPayload := func(title string) []byte {
+		body, _ := json.Marshal(map[string]interface{}{
+			"title":      title,
+			"author":     "First Author",
+			"isbn":       "9780306406157",
+			"library_id": libID,
+			"file_path":  "/test/dup-target.epub",
+			"file_size":  1024,
+			"mime_type":  "application/epub+zip",
+		})
+		return body
+	}
+
+	post := func(body []byte) (*http.Response, []byte) {
+		req, _ := http.NewRequest("POST", setup.Server.URL+"/api/media-items", bytes.NewBuffer(body))
+		req.Header.Set("Content-Type", "application/json")
+		req.Header.Set("Authorization", "Bearer "+token)
+		resp, err := (&http.Client{}).Do(req)
+		require.NoError(t, err)
+		defer func(Body io.ReadCloser) {
+			_ = Body.Close()
+		}(resp.Body)
+		raw, err := io.ReadAll(resp.Body)
+		require.NoError(t, err)
+		return resp, raw
+	}
+
+	resp, raw := post(createPayload("Original Title"))
+	require.Equal(t, http.StatusCreated, resp.StatusCode, "first create should be 201")
+	var created map[string]interface{}
+	require.NoError(t, json.Unmarshal(raw, &created))
+	originalID := created["id"].(string)
+
+	resp, raw = post(createPayload("Different Title"))
+	require.Equal(t, http.StatusOK, resp.StatusCode, "duplicate create should be 200, not a 201 lie")
+
+	var dup map[string]interface{}
+	require.NoError(t, json.Unmarshal(raw, &dup))
+	assert.Equal(t, true, dup["duplicate"], "response must flag the duplicate")
+	assert.Equal(t, originalID, dup["item"].(map[string]interface{})["id"].(string),
+		"duplicate response carries the existing item")
+	assert.Equal(t, "Original Title", dup["item"].(map[string]interface{})["title"],
+		"the second create's payload must NOT overwrite the existing row")
+}
+
 // TestMediaItemLibraryRequirement tests that media items require a library
 func TestMediaItemLibraryRequirement(t *testing.T) {
 	setup := setupTestServer(t)