fix(matrix): preserve exception tracebacks on E2EE and auth failures

Five ``except Exception as exc:`` blocks in the Matrix adapter logged
only ``str(exc)`` without ``exc_info=True``:

- _reverify_keys_after_upload → post-upload key verification failure
- _upload_keys_if_needed      → initial device-key query failure
- _upload_keys_if_needed      → re-upload device keys failure
- _upload_keys_if_needed      → initial device key upload failure
- connect → whoami / access-token validation failure

The E2EE key paths here are security-critical: a silent traceback-
less failure during device-key verification or upload makes it
hard for operators to tell whether their Matrix bot is failing
because of a stale token, a federation timeout, or an olm state
mismatch — all three fail with different tracebacks, which
``str(exc)`` alone flattens.

The contributing guide asks for ``exc_info=True`` on error logs.
Append it to each of the five call sites. Pure logging enrichment.

gateway/platforms/matrix.py

@@ -329,7 +329,7 @@ class MatrixAdapter(BasePlatformAdapter):
                     )
                     return False
         except Exception as exc:
-            logger.error("Matrix: post-upload key verification failed: %s", exc)
+            logger.error("Matrix: post-upload key verification failed: %s", exc, exc_info=True)
             return False
         return True
 
@@ -345,6 +345,7 @@ class MatrixAdapter(BasePlatformAdapter):
             logger.error(
                 "Matrix: cannot verify device keys on server: %s — refusing E2EE",
                 exc,
+                exc_info=True,
             )
             return False
 
@@ -359,7 +360,7 @@ class MatrixAdapter(BasePlatformAdapter):
             try:
                 await olm.share_keys()
             except Exception as exc:
-                logger.error("Matrix: failed to re-upload device keys: %s", exc)
+                logger.error("Matrix: failed to re-upload device keys: %s", exc, exc_info=True)
                 return False
             return await self._reverify_keys_after_upload(client, local_ed25519)
 
@@ -399,6 +400,7 @@ class MatrixAdapter(BasePlatformAdapter):
                     "Try generating a new access token to get a fresh device.",
                     client.device_id,
                     exc,
+                    exc_info=True,
                 )
                 return False
             return await self._reverify_keys_after_upload(client, local_ed25519)
@@ -468,6 +470,7 @@ class MatrixAdapter(BasePlatformAdapter):
                 logger.error(
                     "Matrix: whoami failed — check MATRIX_ACCESS_TOKEN and MATRIX_HOMESERVER: %s",
                     exc,
+                    exc_info=True,
                 )
                 await api.session.close()
                 return False