docs(6206): app.py startup comments state the hazard, not the outage

The deferred-backfill and inline-release-defaults comments describe the
lifespan-overrun hazard they guard without the dated incident; backfill
passes say what they fill rather than which milestone added the vectors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/app.py

@@ -187,12 +187,12 @@ def create_app() -> Quart:
         start_notification_loop()
         start_auth_token_retention_loop()
 
-        # Write down any shipped default that moved in this release (#4225).
+        # Write down any shipped default that moved in this release.
         #
         # INLINE AND AWAITED, deliberately, against the instinct the block
-        # below argues for. What #4181 cost three hours was CONCURRENCY — a
-        # background task created here racing this hook for the same pool.
-        # Running sequentially creates no such contention, and this is twelve
+        # below argues for. The startup hazard is CONCURRENCY — a background
+        # task created here racing this hook for the same pool. Running
+        # sequentially creates no such contention, and this is a dozen
         # single-row reads on an index against a table with a handful of rows.
         #
         # It has to finish before serving because the thing it protects is a
@@ -216,16 +216,14 @@ def create_app() -> Quart:
 
         # Backfill embeddings for any notes that don't have one. Runs in the
         # background so it never blocks the server from accepting requests —
-        # and, since #4181, not until the rest of this hook has finished.
+        # and not until the rest of this hook has finished.
         #
-        # "Background" was true of REQUESTS and false of STARTUP. A task
+        # "Background" is true of REQUESTS and false of STARTUP. A task
         # created here begins immediately, while `before_serving` is still
-        # running, so its four passes competed with the startup hook's own
+        # running, so its passes would compete with the startup hook's own
         # database reads for the same connection pool. That is survivable on a
-        # healthy disk and fatal on a sick one: on 2026-09-19 both this
-        # backfill's first query and the hook's maintenance-hour read were
-        # cancelled together when Hypercorn killed the worker at its lifespan
-        # timeout, and the instance stayed down for three hours.
+        # healthy disk and fatal on a sick one: Hypercorn kills a worker whose
+        # lifespan hook overruns, and nothing retries it.
         #
         # The wait is on the SERVING FLAG rather than a sleep, because a sleep
         # would be a guess about how long the rest of the hook takes and would
@@ -236,21 +234,20 @@ def create_app() -> Quart:
                 await backfill_note_embeddings()
             except Exception:
                 logger.warning("Embedding backfill failed", exc_info=True)
-            # Rules got vectors in milestone 307; every rule written before it
-            # has none, so this is the pass that makes them findable at all.
+            # A rule with no vector is unfindable by meaning; this pass fills
+            # any that are missing.
             try:
                 await backfill_rule_embeddings()
             except Exception:
                 logger.warning("Rule embedding backfill failed", exc_info=True)
-            # Milestones got vectors in milestone 415, so a plan written before
-            # it is findable only after this pass.
+            # Likewise a milestone: a plan with no vector is findable only
+            # after this pass.
             try:
                 await backfill_milestone_embeddings()
             except Exception:
                 logger.warning("Milestone embedding backfill failed", exc_info=True)
-            # Systems got vectors in #4251, so before this pass every charter
-            # ever written is unfindable — a System could narrow a search and
-            # never be the answer to one.
+            # A System charter with no vector is unfindable — the System could
+            # narrow a search and never be the answer to one.
             try:
                 await backfill_system_embeddings()
             except Exception:
@@ -270,7 +267,7 @@ def create_app() -> Quart:
         # work overlaps startup's own.
         asyncio.create_task(_delayed_backfill())
 
-        # RELEASED IN A `finally`, never after the work (rules 156 and 157).
+        # RELEASED IN A `finally`, never after the work.
         # The waiter above has no deadline of its own, and the way to make an
         # undeadlined wait safe is to make the release unmissable: if anything
         # below raises, the flag is still set and the task ends instead of
@@ -292,7 +289,7 @@ def create_app() -> Quart:
 
             # DB maintenance scheduler (daily targeted VACUUM ANALYZE, default
             # 04:00 UTC). `get_maintenance_hour` is the FIRST database read in
-            # this hook and is bounded for that reason (#4181) — see the long
+            # this hook and is bounded for that reason — see the long
             # comment above `_STARTUP_READ_TIMEOUT`. Anything added here that
             # touches the database needs the same treatment: a lifespan hook
             # that does not return is a worker that never serves.
@@ -304,8 +301,8 @@ def create_app() -> Quart:
                 asyncio.get_running_loop(), await get_maintenance_hour()
             )
 
-            # Scheduled backups (milestone 523). An hourly tick that reads its
-            # config when it fires, so starting it reads nothing here (#4181).
+            # Scheduled backups. An hourly tick that reads its config when it
+            # fires, so starting it reads nothing here.
             from scribe.services.backup_job import start_backup_scheduler
             start_backup_scheduler(asyncio.get_running_loop())
 
@@ -315,7 +312,7 @@ def create_app() -> Quart:
             from scribe.services.diagnostics import start_diagnostics
             start_diagnostics(asyncio.get_running_loop())
         finally:
-            # STARTUP IS OVER — release the backfill (#4181). Everything above
+            # STARTUP IS OVER — release the backfill. Everything above
             # is work the server needs done before it serves; everything the
             # backfill does is work that can wait for a server already up.
             logger.info("Startup complete; releasing deferred backfill")