feat!: remove direct messaging (chat)

E2EE direct messages, the chat dock, the Messages module, the message
WebSocket and its realtime registry, and the messages / message_keys
tables (migration 0097; the downgrade restores the empty schema). Prod
held two welcome notes, no message between people and one key — never
used (owner decision, E34). WebSocket now has no owner module: the lint
bans it everywhere. Privacy policy updated: it no longer promises E2EE
for a feature that is gone.

backend/app/auth.py

@@ -278,8 +278,7 @@ ACTIVE_ACCOUNT_HEADER = "x-siftlode-account"
 
 def resolved_user_id(conn: HTTPConnection, *, strict: bool = True) -> tuple[int | None, bool]:
     """Which account this connection acts as, and whether that's the session's default account.
-    Takes any HTTPConnection — an HTTP Request OR a WebSocket — so the WS push channel shares this
-    exact per-tab resolution instead of re-implementing it.
+    Takes an HTTPConnection rather than a Request: it reads only the session and the headers.
 
     The signed cookie holds the *wallet* (`account_ids`, every account signed into this browser)
     plus a default `user_id`. A caller may name its account with the X-Siftlode-Account header (or
@@ -297,8 +296,8 @@ def resolved_user_id(conn: HTTPConnection, *, strict: bool = True) -> tuple[int
     """
     default_id = conn.session.get("user_id")
     wallet = conn.session.get("account_ids") or []
-    # Header for normal XHR; ?account= for contexts that can't set headers (WebSocket, and a
-    # plain <a> file download). Both are wallet-gated below.
+    # Header for normal XHR; ?account= for a context that can't set headers (a plain <a> file
+    # download). Both are wallet-gated below.
     hdr = conn.headers.get(ACTIVE_ACCOUNT_HEADER) or conn.query_params.get("account")
     if hdr:
         try: