## Problem
Scaletest follow-up storms showed that the chat stream path was doing a
same-replica DB reread for every durable message it had already
delivered locally.
In a 600-chat / 10-turn run, `/stream`-attributed
`GetChatMessagesByChatID` calls reached about 14.2k across 5,400
follow-up turns — roughly **2.63 rereads per turn**. The primary coderd
replicas saturated their DB pools at 60/60 open connections during the
storm window.
The root cause: when pubsub was active, `Subscribe()` suppressed local
durable `message` events and relied entirely on pubsub notify →
`GetChatMessagesByChatID` for catch-up. Same-replica subscribers paid
the full DB round-trip even though the persisting process was on the
same replica.
## Solution
Add a bounded per-chat **durable message cache** to `chatStreamState` so
that same-replica subscribers can catch up from memory instead of the
database.
### How it works
1. `publishMessage()` caches the SDK event in `chatStreamState` before
local fanout and pubsub notify.
2. `publishEditedMessage()` replaces the cache with only the edited
message, then publishes `FullRefresh`.
3. `Subscribe()` handles ordinary `AfterMessageID` notifies by first
consulting the per-chat durable cache and only falling back to
`GetChatMessagesByChatID` on cache miss.
4. `FullRefresh` always forces a DB reread (cache is bypassed).
### Safety properties
- If the cache misses (e.g. message expired or remote replica), the DB
catch-up still runs — no silent message loss.
- `FullRefresh` (edits) always rereads from the database.
- Remote replicas still use the pubsub + DB path unchanged.
- The cache is bounded (`maxDurableMessageCacheSize = 256`) and scoped
per chat — no unbounded memory growth.
## Impact
This change removes the entire same-replica portion of the stream
rereads. Based on the 600-chat follow-up run, the upper bound on saved
work is the same-replica share of about 14.2k `GetChatMessagesByChatID`
rereads, with the observed total stream reread rate at about 2.63
rereads per follow-up turn.
Ethan
cda460f5df