Add changelog entries for weeks of April 13 & April 20, 2026 (#1032)

* docs(whats-new): add Week of April 20, 2026 entry

Adds a new changelog entry covering logs UX refresh, Squads
`previousAssistantMessages` handoff type, `assistant.speechStarted`
GA with word-level timing, and transcriber autofallbacks.

https://claude.ai/code/session_01LZKc1zrMXuAC3GXiGc4Gin

* docs(whats-new): add Week of April 13, 2026 entry

Announces Monitoring GA with links to the quickstart docs and
announcement blog post.

https://claude.ai/code/session_01LZKc1zrMXuAC3GXiGc4Gin

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: bets-margulies-vapi

fern/changelog/2026-04-13.mdx

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+# What's New: Week of April 13, 2026
+
+1. **Monitoring — GA**: Automated call quality monitoring is now generally available. Detect issues with trigger-based rules, get alerts when something goes wrong, and surface resolution suggestions — all from the dashboard.
+
+    - [Monitoring quickstart](https://docs.vapi.ai/observability/monitoring-quickstart)
+    - [Announcement blog post](https://blog.vapi.ai/monitoring)

fern/changelog/2026-04-20.mdx

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+# What's New: Week of April 20, 2026
+
+1. **Logs UX Refresh**: New filter layout plus a round of UX improvements — improved date picker, active row is clearly highlighted across all log views when the flyout is opened, log tables are fully keyboard-accessible, sortable `cost` and `duration` columns, pagination, and more.
+
+2. **Squads `contextEngineeringPlan` Handoff Type — `previousAssistantMessages`**: Forwards only the conversation history from *before* the current assistant's session. The current assistant's own messages and tool calls are excluded entirely from the handoff payload. See the updated [handoff context configuration docs](https://docs.vapi.ai/security-and-privacy/pci#handoff-context-configuration).
+
+3. **`assistant.speechStarted` Event — Live Captions & Word-Level Timing (GA)**: A new opt-in message fires as the assistant begins speaking each segment, carrying the full turn text, `turn`, `source` (`model` / `force-say` / `custom-voice`), and optional timing:
+    - Per-word alignment on **ElevenLabs**
+    - Cursor-based word-progress on **Minimax** (set `voice.subtitleType: "word"`, with correct CJK handling)
+    - Text-only fallback on all other providers
+
+    Subscribe by adding `"assistant.speechStarted"` to your assistant's `clientMessages` and/or `serverMessages` — now **GA with no feature flag**. Use it for live captions, karaoke-style highlighting, or any UI that needs to stay in sync with assistant audio. Fully backward-compatible; no existing messages changed.
+
+4. **Autofallbacks on Transcribers**: Let Vapi pick the best transcriber to fall back to if your primary one fails — even mid-call. Opt in by setting `assistant.transcriber.fallbackPlan.autoFallback.enabled` to `true`. See the updated [transcriber fallback plan docs](https://docs.vapi.ai/customization/transcriber-fallback-plan).