fix(mcp): disable DNS-rebinding protection on FastMCP

FastMCP defaults to an allow-list of localhost variants for the Host
header (DNS-rebinding protection). Any deployment behind a reverse
proxy hitting a non-localhost hostname (e.g. devassistant.traefik.internal)
gets 421 Misdirected Request with:

  WARNING mcp.server.transport_security: Invalid Host header: <name>

The protection exists to stop a malicious browser page from rebinding
DNS to attack a localhost MCP server. Our deployment is HTTP transport
behind a reverse proxy with bearer-token auth, which already gates
every request — so the rebinding threat doesn't apply. Disabling
the check lets any Host through; auth still rejects unauthorized
requests at 401.

This also makes the integration test pass without test-only host
hackery — every realistic Host header (Traefik internal hostname,
CDN domain, custom DNS) now reaches FastMCP cleanly.

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

src/fabledassistant/mcp/server.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 from mcp.server.fastmcp import FastMCP
+from mcp.server.transport_security import TransportSecuritySettings
 from quart import Quart
 
 _INSTRUCTIONS = """
@@ -23,8 +24,24 @@ Hierarchy: Project -> Milestone -> Task/Note.
 
 
 def build_mcp_server() -> FastMCP:
-    """Build the FastMCP instance with all tools registered."""
-    mcp = FastMCP("scribe", instructions=_INSTRUCTIONS.strip())
+    """Build the FastMCP instance with all tools registered.
+
+    DNS-rebinding protection is disabled: FastMCP's default allow-list is
+    just localhost variants, which means any deployment behind a reverse
+    proxy (Traefik with a hostname like devassistant.traefik.internal,
+    Cloudflare, nginx, etc.) gets 421 Misdirected Request. The threat
+    model that protection addresses — a malicious browser page rebinding
+    DNS to hit a localhost MCP — doesn't apply here: this is HTTP transport
+    behind a reverse proxy with bearer-token auth as the real security
+    boundary.
+    """
+    mcp = FastMCP(
+        "scribe",
+        instructions=_INSTRUCTIONS.strip(),
+        transport_security=TransportSecuritySettings(
+            enable_dns_rebinding_protection=False,
+        ),
+    )
     from fabledassistant.mcp.tools import register_all
     register_all(mcp)
     return mcp