chore: relax conversation meta polling for high-volume accounts (#14518)

On high-volume accounts, the dashboard sidebar's conversation count
badges fall behind because a meaningful share of
`/api/v1/accounts/:id/conversations/meta` requests get rate-limited
(per-user throttle, default 30 req/min).

Root cause is in `conversationStats.js`. The tiered debounce uses
`allCount` from the last response to pick a wait interval. `allCount`
reflects the user's *current filtered scope*, not the account's true
volume — so an agent viewing a small filter on a busy account falls into
the most aggressive tier (500ms wait / 1.5s maxWait → up to 40
calls/min/tab) and trips the throttle.

## What changed

`app/javascript/dashboard/store/modules/conversationStats.js`:

- Short-tier `maxWait`: `1500 → 2000` (caps short-tier at 30/min/tab
instead of 40)
- Super-long-tier threshold: `allCount > 5000 → > 2000` (more
high-volume accounts fall into the safe 3/min/tab tier)
- Middle-tier threshold unchanged (`> 100`)

| Tier (allCount) | wait / maxWait | Calls/min/tab |
|---|---|---|
| `> 2000` | 10s / 20s | 3 |
| `> 100` | 5s / 10s | 6 |
| else | 500ms / 2s | 30 |

## Trade-off

Badge updates (including those triggered by the agent's own action) may
lag by up to the tier's `maxWait` — worst case 20s for accounts with >
2000 open conversations in the active scope. The conversation list
itself and push notifications continue to update in real time; only the
numeric badge is debounced.

## Not in scope

- Sticky-max `allCount` to fix the underlying tier-selection signal —
defer until the simpler tuning is validated in production
- Optimistic count updates on local user actions — adds non-trivial
state management for a cosmetic lag

app/javascript/dashboard/store/modules/conversationStats.js

@@ -25,7 +25,7 @@ const fetchMetaData = async (commit, params) => {
   }
 };
 
-const debouncedFetchMetaData = debounce(fetchMetaData, 500, false, 1500);
+const debouncedFetchMetaData = debounce(fetchMetaData, 500, false, 2000);
 const longDebouncedFetchMetaData = debounce(fetchMetaData, 5000, false, 10000);
 const superLongDebouncedFetchMetaData = debounce(
   fetchMetaData,
@@ -36,7 +36,7 @@ const superLongDebouncedFetchMetaData = debounce(
 
 export const actions = {
   get: async ({ commit, state: $state }, params) => {
-    if ($state.allCount > 5000) {
+    if ($state.allCount > 2000) {
       superLongDebouncedFetchMetaData(commit, params);
     } else if ($state.allCount > 100) {
       longDebouncedFetchMetaData(commit, params);