docs(changelog): stamp #6979 per-message media preview snapshots

Co-authored-by: happy5318 <happy5318@users.noreply.github.com>

CHANGELOG.md

@@ -5,6 +5,8 @@
 
 ### Fixed
 
+- **Historical chat media previews no longer change when a file is overwritten in place.** After the recent switch to revalidating `/api/media` responses, overwriting a file with the same name (e.g. regenerating `chart.png`) silently rewrote every earlier chat preview that referenced it, so you lost the ability to compare the old and new versions. Each settled message now freezes the bytes of its local-file previews into a content-addressed snapshot store and serves those frozen bytes for that message, while new turns still show the current file. The snapshot store is internal (never reachable as a normal media path), bounded (50 MB per file, 2 GB total, oldest-evicted-first), and each stored digest is bound to the exact source path it was captured from, so a snapshot can only ever be served for its own message. Thanks @happy5318. (#6979)
+
 - **Multi-container Docker deployments can reach the agent API again.** In the two- and three-container Compose topologies, the agent's gateway API bound to loopback only, so the WebUI (and dashboard) containers — separate hosts on the Docker network — couldn't reach `http://hermes-agent:8642` even with `HERMES_API_URL` and `API_SERVER_KEY` set correctly, breaking cron/Tasks and gateway-backed features. The agent service now sets `API_SERVER_HOST=0.0.0.0` so it's reachable across the Compose network; the host port publish stays `127.0.0.1`-only, so this does not expose the API beyond the Docker network and host loopback. Thanks @bragov4ik. (#6968)
 
 - **Two WebUI sessions with the same auto-generated title no longer leave the second one blank.** Follow-up to the `state.db` title sync: hermes-agent enforces a uniqueness rule on session titles, so when two sessions independently generated a byte-identical auto-title, the second sync raised (and silently swallowed) a collision error and left that session untitled in `hermes sessions list`. The sync now catches the collision, derives a de-duplicated variant via the existing lineage helper (`"My Session"` → `"My Session #2"`), and retries — and because the retry goes through the same provenance guard, a title you set manually is still never overwritten. Thanks @webtecnica. (#6964)