fix(webapp): correct backward pagination slice on session list

Devin catch on #3417 — the ClickHouse sessions list was slicing
`sessionIds.slice(1, size + 1)` on the backward path, which skipped
the item closest to the cursor and surfaced the sentinel (the
`size+1`th item that proves hasMore=true) to the user.

Trace, with items c01…c11 and cursor=c07 (page size 3):
- Backward query: `session_id > c07 ORDER BY ASC LIMIT 4` →
  `[c08, c09, c10, c11]`. Legitimate content is the first three
  (`[c08, c09, c10]`); `c11` is the sentinel.
- Previous slice: `[c09, c10, c11]` → displayed DESC `[c11, c10, c09]`
  — user never sees c08, sees sentinel c11 instead.

Fix: collapse both directions to `sessionIds.slice(0, size)`. The
sentinel is always the last item regardless of direction, so the two
branches had no reason to diverge. Cursor computations
(`previousCursor = reversed.at(1)`, `nextCursor = reversed.at(size)`)
already line up with the corrected slice — no change needed there.

Verified: webapp typecheck clean.

Author: ericallam

apps/webapp/app/services/sessionsRepository/clickhouseSessionsRepository.server.ts

@@ -74,10 +74,12 @@ export class ClickHouseSessionsRepository implements ISessionsRepository {
       }
     }
 
-    const idsToReturn =
-      options.page.direction === "backward" && hasMore
-        ? sessionIds.slice(1, options.page.size + 1)
-        : sessionIds.slice(0, options.page.size);
+    // Both directions slice the first `size` IDs: the `size+1`th item is
+    // the sentinel proving another page exists (hasMore), not part of the
+    // page content. Backward queries sort ASC (items closest to the cursor
+    // first), so `[0..size)` is still the legitimate window and the last
+    // element is the sentinel — identical to the forward case.
+    const idsToReturn = sessionIds.slice(0, options.page.size);
 
     let sessions = await this.options.prisma.session.findMany({
       where: {