* feat(mcp): add Bearer token authentication support
Add HINDSIGHT_API_MCP_AUTH_TOKEN environment variable to enable
authentication for MCP endpoint. When set, all requests must include
a valid Authorization header (Bearer token or direct token).
If not set, MCP endpoint remains open for backwards compatibility
with local development environments.
* fix: propagate Bearer token from MCP middleware to tools for tenant auth
MCP tools were creating RequestContext() without api_key, causing
"Invalid API key" errors when tenant extension validates requests.
Now the Bearer token is extracted in middleware, stored in a context
variable, and passed through to all MCP tool RequestContext instances.
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