docs(onboard): BDD scenarios + architecture for the ADR-028 burst

- connect_account.feature: refine the burst scenario to "best recent" (likely-good
  selection, skips too-short/too-long) and add a scenario for the stronger model.
- scenario_coverage: remap to TestOnboardBurstVideoIDs + add the model scenario
  (the old NewestUnsummarizedVideoIDs test was removed).
- architecture.md: new "Connect-time onboarding burst" subsection — junk-avoiding
  selection over persisted duration, the burst-only stronger-model chain, and why
  has-captions/cached-first are not selection signals.

docs/architecture/architecture.md

@@ -250,6 +250,32 @@ After  (newest-first):       `[chanB-new, chanA-mid, chanA-old, chanB-null]`
 The set of *processed* videos now also excludes auto-mode back-catalogue beyond the recency
 window (those stay listed, summarised on demand); within the processed set, only order changes.
 
+### Connect-time onboarding burst (ADR-018 → ADR-028)
+
+On a successful YouTube connect, `ConnectHandler` enqueues a connect-triggered discovery pass;
+the `discoveryTrigger` runs that pass and then fires the **onboarding burst** — a third entry path
+that summarises up to `TAPIR_ONBOARD_SUMMARIZE_COUNT` (default 3, hard-capped) of the new user's
+videos so the first session is not empty. The burst still flows through `globalFetchGate` (it is
+not a throughput change); ADR-028 sharpened *which* videos and *which model*:
+
+- **Selection** is `OnboardBurstVideoIDs`, not pure newest-first. It keeps newest-first order but
+  excludes a video whose **known** duration is outside `[TAPIR_MIN_VIDEO_SECONDS,
+  TAPIR_ONBOARD_MAX_VIDEO_SECONDS]` (drops Shorts and multi-hour livestream VODs). An unknown
+  (NULL) duration is degrade-open — kept, but ranked after known-good rows. The connect-triggered
+  discovery pass runs *before* the burst, and ADR-023's `videos.list` enrichment now **persists**
+  `duration_s` (instead of discarding it after the Shorts filter), so a fresh user's candidates
+  carry a duration in time for selection.
+- **Model**: the burst runs through a dedicated summarizer chain led by
+  `TAPIR_ONBOARD_SUMMARIZER_MODEL` (default `iguana/gemma4-26b`, the stronger local model), with
+  the standard ADR-022 chain following as fallback. This is a wiring choice — a second
+  `engineProcessor` over the same store / transcript cache / sink; the engine and ports are
+  unchanged. Empty / equal-to-primary collapses it back onto the shared processor.
+
+`has-captions` is deliberately **not** a selection signal — it is only knowable after a gate fetch
+(or a ~0-probability cache hit at pilot scale), so the burst can avoid known-junk but cannot
+promise captions. Cached-transcript-first selection was investigated and rejected (ADR-028:
+~3% cross-user overlap).
+
 ---
 
 ## Sequence — core use case: new video summarized

docs/use-cases/connect_account.feature

@@ -24,12 +24,19 @@ Feature: Connect and manage video accounts
     Then a discovery pass for my account is triggered right away
     And I do not have to wait for the next scheduled pass to see my videos
 
-  Scenario: Connecting summarizes my newest videos right away
+  Scenario: Connecting summarizes my best recent videos right away
     Given I have no connected video accounts
     When I connect my YouTube account
-    Then up to the onboarding cap of my newest videos are summarized through the rate gate
+    Then up to the onboarding cap of my newest likely-good videos are summarized through the rate gate
+    And videos whose known duration is too short or too long are skipped
     And the rest are left to the scheduled recency-bounded pass
 
+  Scenario: The onboarding burst summarizes with a stronger model
+    Given I have no connected video accounts
+    When I connect my YouTube account
+    Then the burst summarizes with the stronger onboarding model first
+    And the standard summarizer chain still follows as a fallback
+
   Scenario: Tokens are never stored in the clear
     When I connect any video account
     Then no OAuth token value is stored in the database

test/acceptance/scenario_coverage_test.go

@@ -38,10 +38,11 @@ var scenarioCoverage = map[string]string{
 	"An authenticated user on the welcome page sees their way in and out": "TestWelcomeLoggedIn",
 
 	// connect_account.feature
-	"Connect a YouTube account":                          "TestCallbackExchangesAndRecordsConnection",
-	"Connecting an account discovers videos immediately": "TestCallbackTriggersDiscovery",
-	"Connecting summarizes my newest videos right away":  "TestNewestUnsummarizedVideoIDs",
-	"Tokens are never stored in the clear":               "TestCallbackExchangesAndRecordsConnection",
+	"Connect a YouTube account":                              "TestCallbackExchangesAndRecordsConnection",
+	"Connecting an account discovers videos immediately":     "TestCallbackTriggersDiscovery",
+	"Connecting summarizes my best recent videos right away": "TestOnboardBurstVideoIDs",
+	"The onboarding burst summarizes with a stronger model":  "TestBurstChainModelsLeadsWithOnboardModel",
+	"Tokens are never stored in the clear":                   "TestCallbackExchangesAndRecordsConnection",
 
 	// paste_url.feature
 	"Paste a valid YouTube URL":                              "TestPasteValidURLAddsAndRequests",