feat: unique display names (case-insensitive) + inline recover on join

Backend:
- Migration 007 deduplicates existing users by name (case-insensitive,
  keeping oldest) and adds a unique index on (event_id, LOWER(display_name))
- find_by_event_and_name is now case-insensitive (LOWER comparison)
- join endpoint checks name availability before creating; returns 409
  Conflict with code "conflict" when the name is taken

Frontend (join page):
- On 409, switches to a name-taken view showing an amber warning with
  name-choice tips, a PIN input that runs the recover flow inline, and
  an "Anderen Namen wählen" button that returns to the name input

Test guide: added Steps 8 (rate-limit auto-retry) and 9 (name uniqueness)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TEST_GUIDE.md

@@ -40,3 +40,18 @@ Please test each step in order and report any errors (console errors, wrong text
 23. Open a new **private/incognito** window at **http://localhost:5173/recover**
 24. Enter the same name (`Max`) and the PIN you copied
 25. ✅ Expected: You're redirected to the feed with the same account
+
+### Step 8 — Upload rate-limit auto-retry
+26. Upload more than 20 photos in one hour to trigger the rate limit
+27. ✅ Expected: When the limit is hit, remaining items stay **Wartend** (not error)
+28. ✅ Expected: An amber banner appears in the queue: "Upload-Limit erreicht. Wird in Xs automatisch fortgesetzt."
+29. ✅ Expected: The countdown ticks down and uploads resume automatically when it reaches 0
+
+### Step 9 — Name uniqueness (case-insensitive)
+30. In a private/incognito window go to **http://localhost:5173/join**
+31. Enter `max` or `MAX` — the same name already taken in Step 1 (different case)
+32. ✅ Expected: Instead of creating a new account, an amber warning appears: „Max ist bereits vergeben." with name tips
+33. ✅ Expected: A PIN input and **Anmelden** button appear, plus an **Anderen Namen wählen** button
+34. Enter your PIN from Step 1 and click **Anmelden**
+35. ✅ Expected: You're signed in to the existing `Max` account and redirected to the feed
+36. Alternatively, click **Anderen Namen wählen** — ✅ Expected: the name input reappears with `max` pre-filled so you can edit it