feat(#551): gate server-side e2ee behind opt-in matrix.allowEncryption (default off)

docs/matrix.md

@@ -119,9 +119,16 @@ Initial rollout settings:
   engage). The absent
   `yes_i_am_very_very_sure_…_open_registration_…` flag keeps the
   server closed to anyone without the token.
-- `allow_encryption = true` — E2EE enabled. Agent clients use the
+- `allow_encryption` — server-side E2EE switch, sourced from
-  `e2e-encryption` feature of `matrix-sdk`; crypto keys are stored in
+  `services.hyperhive.matrix.allowEncryption` (**default `false`**, opt-in).
-  the per-agent sqlite store under the state dir and persist across
+  Off by default because on the hive-internal homeserver the operator
+  already controls the transport; turn it on for encrypted rooms on
+  external / federated homeservers or to keep contents opaque to the
+  homeserver admin. **The agent matrix client always supports decryption
+  regardless of this flag** — it uses the `e2e-encryption` feature of
+  `matrix-sdk` so it can read encrypted rooms it is invited to even when
+  this homeserver doesn't permit room encryption. Crypto keys are stored
+  in the per-agent sqlite store under the state dir and persist across
   restarts (lost on `--purge`). `read_room` decrypts via
   `room.messages()` — UTD events surface as `event_type =
   "m.room.encrypted"` with `body = "[unable to decrypt]"`.

nix/modules/hive-matrix.nix

@@ -241,6 +241,25 @@ in
       '';
     };
 
+    allowEncryption = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Server-side switch for matrix end-to-end encryption — sets
+        tuwunel's `allow_encryption`. Off by default: on the hive-internal
+        homeserver the operator already controls the transport, so server
+        E2EE adds key-management overhead (cross-signing, device
+        verification, undecryptable-message recovery) without a clear
+        threat-model win for the common single-hive case. Turn on when
+        agents join encrypted rooms on external / federated homeservers,
+        or when the operator wants message contents opaque to the
+        homeserver admin. Independent of the agent matrix client, which
+        always supports decryption so it can read encrypted rooms it is
+        invited to regardless of this flag; this option only governs
+        whether THIS homeserver permits room encryption.
+      '';
+    };
+
     gui = {
       enable = lib.mkOption {
         type = lib.types.bool;
@@ -403,7 +422,9 @@ in
               # LoadCredential below copies the host file into a
               # 0400 dynamic-user-owned path; tuwunel reads from there.
               registration_token_file = "/run/credentials/tuwunel.service/registration_token";
-              allow_encryption = true;
+              # Server-side E2EE is opt-in (default off); the agent matrix
+              # client always supports decryption regardless.
+              allow_encryption = cfg.allowEncryption;
               # Tuwunel's default suffix is " 💕" — suppress it so agent
               # display names are clean (just the agent name, no emoji).
               new_user_displayname_suffix = "";