whatsapp: persist chats and messages across restarts (v8 / 2.2.0)

WhatsApp delivers a history sync at login, not on reconnect, so an in-memory
store left the connector blind after every process restart (23 in two weeks).
The store is now mirrored to store/, next to auth/ and bind-mounted the same
way.

Not SQLite: node:sqlite needs Node 22 (unflagged only from 24) and the runtime
image ships Debian trixie's nodejs = 20.19.2; better-sqlite3 is native and the
slim image has no toolchain. So an append-only JSONL log for messages plus a
debounced JSON snapshot for chats/contacts, both written tmp+rename so a crash
cannot truncate them. Compaction on load and every 500 appends keeps the log
from creeping upward.

Also fixes a pre-existing duplication bug: pushMessage appended unconditionally,
re-adding every message a history re-sync redelivered. It now returns false on a
known id and the message is skipped in both the transcript and the log.

MAX_MSGS_PER_CHAT 200 -> 500. logout deletes store/ with auth/, so re-linking a
different phone cannot inherit the previous account's history.

Data at rest: message text is now written to the user's bind-mounted home.
Nothing but session keys was persisted before.

Verified on skald-runtime:v4 with a seeded store: 751 lines with 50 duplicates,
a 700-message chat and a torn trailing line -> 701 loaded, compacted to 501, cap
applied, second run 501 -> 501 unchanged.

connectors/whatsapp/fragment.json

@@ -20,7 +20,7 @@
     "type": "qr"
   },
   "folder": "whatsapp",
-  "version": 7,
-  "version_string": "2.1.0",
+  "version": 8,
+  "version_string": "2.2.0",
   "version_release_date": "2026-08-23"
 }