fix: harden MCP endpoint after code review

Critical: the MCP SDK auto-enables DNS-rebinding protection when its host
parameter is left at the 127.0.0.1 default, hard-coding a loopback-only Host
allow-list. Every remote client (the only deployment this feature targets) was
refused with HTTP 421 before auth or the tool ran. Now driven by a new
MCP_ALLOWED_HOSTS setting, with '*' as an explicit, logged opt-out.

Also bounds query/top_k to match AskRequest (top_k is amplified 4x downstream,
so an unbounded value was a resource-exhaustion vector), decodes the
Authorization header as latin-1 per the ASGI spec instead of raising a 500 on
malformed bytes, and returns WWW-Authenticate on 401 per RFC 7235.

Moves the psycopg2 import guard into backend/tests/conftest.py: duplicated
across four test modules, it only worked because of alphabetical collection
order, and any earlier-sorting package would have reintroduced a live
connection attempt against the production database.

Registers the mcp module in the authoritative backend architecture doc.

84 backend tests pass. Verified against a live server: allowed remote Host
returns a valid initialize result, unknown Host returns 421, missing token
returns 401 with WWW-Authenticate.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/perception/test_postgres_event_store.py

@@ -4,14 +4,8 @@ import json
 from unittest.mock import MagicMock, patch
 import pytest
 
-# Patch psycopg2 before importing the module under test
-import sys
-mock_psycopg2 = MagicMock()
-mock_psycopg2.extras = MagicMock()
-sys.modules.setdefault("psycopg2", mock_psycopg2)
-sys.modules.setdefault("psycopg2.extras", mock_psycopg2.extras)
-sys.modules.setdefault("psycopg2.pool", MagicMock())
-
+# psycopg2 is mocked centrally in backend/tests/conftest.py, so importing the
+# module under test here never binds the real driver.
 from app.infrastructure.perception.base_event_store import BaseEventStore