feat(talk): add MCP integration for Nextcloud Talk (spreed)

Adds 6 MCP tools so an LLM can read a user's Talk conversations and
post messages on their behalf, addressing the "read my chats and reply"
use case from issue #720:

  - talk_list_conversations
  - talk_get_conversation
  - talk_get_messages
  - talk_list_participants
  - talk_send_message    (auto-attaches a referenceId for retry dedup)
  - talk_mark_as_read

Edit/delete messages, reactions, threads, and call/session ops are
intentionally out of scope for this first PR.

The TalkClient also exposes create_conversation/delete_conversation
for the integration test fixture; these are not registered as MCP
tools. A post-installation hook enables spreed in the docker dev env
so the integration suite has a real Talk backend to talk to.

Closes #720

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/scope_authorization.py

@@ -209,6 +209,7 @@ def require_scopes(*required_scopes: str):
                             "files.",
                             "tables.",
                             "deck.",
+                            "talk.",
                         ]
                     )
                 ]
@@ -229,6 +230,7 @@ def require_scopes(*required_scopes: str):
                             "files.",
                             "tables.",
                             "deck.",
+                            "talk.",
                         ]
                     )