docs(architecture): add in-process scheduler sequence, rate gate description, auto_summarize default fix

docs/architecture/architecture.md

@@ -145,8 +145,9 @@ graph TB
   engine in a **background goroutine** inside `serve`; the page HTMX-polls `/v/{id}/status`,
   showing a Charmbracelet spinner while in-flight (and an honest "queued/waiting" state under
   rate-limiting — ADR-014).
-- **Summarization mode** — `users.auto_summarize` (migration 006). Auto: every new video is
-  summarized. Manual (default): new videos appear unsummarized; the button sets
+- **Summarization mode** — `users.auto_summarize` (migration 006). Default is **true** for new
+  users (migration 011, ADR-018); all existing rows were back-filled via migration 012. Auto:
+  every new video is summarized. Manual: new videos appear unsummarized; the button sets
   `videos.summarize_requested`, which the next `tapir run` processes and clears. Both the click
   path and the batch `tapir run` drive the same unchanged engine.
 
@@ -155,6 +156,60 @@ reads the store and triggers the existing engine. Adding it changed wiring, not
 
 ---
 
+## In-process scheduler (ADR-018)
+
+`cmdServe` launches a background goroutine when `TAPIR_DISCOVERY_INTERVAL > 0`. On each tick
+it calls `store.ListAllUsers` (un-RLS'd admin query), builds a per-user `runner.Runner`, and
+calls `RunOnce` for each registered user in sequence.
+
+```mermaid
+sequenceDiagram
+    participant S as Scheduler goroutine
+    participant DB as Postgres (RLS)
+    participant YT as YouTube timedtext
+    participant LLM as LiteLLM gateway
+
+    loop every TAPIR_DISCOVERY_INTERVAL
+        S->>DB: ListAllUsers() [un-RLS'd]
+        loop per user
+            S->>DB: GetAutoSummarize(userID)
+            S->>YT: ListSubscriptions + NewVideos
+            Note over S,YT: WaitFetchGate(ctx) throttles<br/>all fetches to TAPIR_FETCH_RATE
+            alt transcript available
+                S->>LLM: Summarize
+                S->>DB: Deliver(summary)
+            else 429
+                S->>DB: SetTranscriptStatus(rate_limited)
+            end
+        end
+    end
+```
+
+**Single-replica constraint (load-bearing).** The scheduler lives in the web process;
+`replicas: 1` in the k3s deployment manifest is not cosmetic — running `tapir serve` at
+>1 replica makes every replica run the full discovery loop, causing every registered user
+to be fetched in parallel from the same egress IP (429s + duplicate work). Do not scale
+`serve` past 1 replica without first moving discovery to a k8s CronJob or adding leader
+election. The process logs a `Warn` at startup when scheduled discovery is enabled as a
+reminder.
+
+---
+
+## Process-wide timedtext rate gate
+
+**`internal/adapters/youtube/gate.go`** (ADR-014 item 2): a single `rate.Limiter`
+(`golang.org/x/time/rate`) shared across **all** Adapter instances. Every `httpDo` call for
+a caption fetch passes through `WaitFetchGate(ctx)` before hitting YouTube. This serialises
+the scheduler loop AND the web click-path through the same per-egress-IP budget. Configured
+via `TAPIR_FETCH_RATE` (Go duration, default `2s`). Setting it to `0` disables the gate
+(dev/tests only).
+
+This is the precondition that makes scheduled auto-summarize safe: without the gate, a
+multi-user scheduler pass could fire many concurrent timedtext requests from the same IP
+within seconds, triggering 429s for all users.
+
+---
+
 ## Sequence — core use case: new video summarized
 
 ```mermaid