feat: assistant overview page [CW-7408] (#14889)

This PR adds a Captain Assistant **Overview** page to show some KPI
metrics (conversations handled, auto-resolution, handoff, hours saved,
reopen-after-resolve, conversation depth) with trend deltas vs the
previous window, a real knowledge card, and a lazily-loaded, cached LLM
welcome summary.

### Highlights

- **Two contextual banners** on the overview:
- **Inbox banner** — prompts the user to connect an inbox when the
assistant has none, so it can actually do work.
- **Coverage banner** — warns when FAQ coverage is below 85% with more
than 100 responses pending review, linking straight to the pending
queue. Dismissal persists per-assistant for 24h via localStorage.
- **Batched stats builder** (`Captain::AssistantStatsBuilder`) computes
both windows in single FILTER-aggregated scans to cut round trips,
behind new `stats`/`summary` endpoints.
- **Cards included but intentionally left dummy / not rendered yet:**
`ResponseQualityCard` (flagged responses) and `CreditUsageCard` (credit
usage + daily chart). Credits are an account-wide counter with no
per-assistant or daily history, so there is no real data to back them
yet; they ship in the codebase but are not wired into the page.

### Index migration

- Replaces `index_messages_on_sender_type_and_sender_id` with
`index_messages_on_sender_and_created` `(sender_type, sender_id,
created_at)`.
- **Why it helps:** the per-assistant windowed lookups filter `sender_*`
*and* a `created_at` range. The old 2-column index matched every
lifetime row for the assistant and filtered the time slice at the heap
(~89% of rows discarded); adding `created_at` as a range column lets
Postgres scan only the window, and fixes the row-count estimate so the
planner picks a hash join over a nested loop on `reporting_events`.
- **Why dropping the old index is safe:** the new index is a left-prefix
superset `(sender_type, sender_id, ...)`, so every query the old one
served is still served. No code references it by name, and dropping it
keeps write amplification on `messages` neutral. Built/dropped with
`CONCURRENTLY` and `if_not_exists`/`if_exists` guards.


## Preview

<img width="2572" height="1754" alt="CleanShot 2026-06-29 at 22 38
51@2x"
src="https://github.com/user-attachments/assets/3798d09e-7850-48e4-b2cd-508533f15cea"
/>

## Banners

#### Inbox connect alert

<img width="2178" height="612" alt="CleanShot 2026-06-30 at 14 26 55@2x"
src="https://github.com/user-attachments/assets/373c371c-bb7d-4291-a0f9-620673078302"
/>

#### Coverage alert
<img width="2178" height="612" alt="CleanShot 2026-06-30 at 14 25 41@2x"
src="https://github.com/user-attachments/assets/e12d6308-11b6-4ba2-88a2-8a3077dd3e8f"
/>

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/javascript/dashboard/helper/featureHelper.js

@@ -19,6 +19,7 @@ const FEATURE_HELP_URLS = {
   webhook: 'https://chwt.app/hc/webhooks',
   billing: 'https://chwt.app/pricing',
   saml: 'https://chwt.app/hc/saml',
+  captain: 'https://chwt.app/captain-docs',
   captain_billing: 'https://chwt.app/hc/captain_billing',
 };