Allow empty places in rankings; rebuild the list editor

Entries now sit at places 1…max with gaps allowed (#28, nothing at #29,
#30). Moving onto an empty place just puts the entry there; onto a taken
place, the entries in between shift one step, stopping at the first
empty place, so lists without gaps reorder exactly as before. Inserts
push down only to the next gap; deleting can close up or leave the place
empty. New endpoint POST /api/lists/{id}/places inserts or removes empty
places or closes every gap; move and delete reply with every entry's
place. The podium shows an empty step for an empty #2 or #3, and a list
can't shrink below its last used place.

The editor loads every entry (no paging) and applies edits instantly
with the same rules (places.js mirrors places.go), queueing saves in the
background and reconciling with the server's reply. Type a place and
press Enter (↑/↓ move between boxes, Alt+↑/↓ moves the entry), drag, or
use the row menu. Empty runs show as rows with Add here and Remove, and
a toolbar shows the gap count, Close gaps and a Go to # box. Clicking an
entry opens it for editing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -131,6 +131,17 @@ A list can hold 1 to 65,535 entries. The editor recommends **100 or fewer** and
 
 To share with friends and family without being fully public, set a password under **Settings → Privacy → Site password**. Visitors then see a password prompt over a blurred placeholder; none of your lists, images or profile are sent until they enter it. Once they do, that browser stays unlocked for 90 days. Changing the password asks everyone for the new one, and you never need it while signed in. A protected site also tells search engines not to index it.
 
+## Ranking
+
+Entries sit at places from #1 to the list's size, and a place can be left empty: a game can be #28 with nothing at #29 and the next at #30. In the list editor:
+
+- **Type a place** in an entry's number box and press Enter (↑/↓ jump between boxes, Alt+↑/↓ moves the entry one place). Or drag it.
+- Moving onto an **empty place** just puts it there. Moving onto a **taken place** shifts the entries in between one step, stopping at the first empty place, so entries you placed further down stay put.
+- An entry's **⋯ menu** can insert an empty place above it, move it to #1 or the end, or remove it (you choose whether the entries below move up or the place stays empty).
+- Empty places show as their own rows: **Add here** makes the next entry you add go there, and **Remove** closes them up. **Close gaps** renumbers everything 1, 2, 3….
+
+Edits show instantly and save in the background.
+
 ## Importing a list
 
 **Import** in the list editor adds entries from a file, after the ones already there and in the file's order: