feat(m7.3): MCP tenant-scoped bearer tokens (#92)

MCP server validates per-tenant bearer tokens on incoming calls and routes each tool to the caller's tenant DB. Closes the cross-tenant data leak in the MCP path identified in M7.3.

compliance-agent/src/database.rs

@@ -141,6 +141,25 @@ impl DatabasePool {
         &self.client
     }
 
+    /// Cross-tenant admin database used by features that intentionally
+    /// span tenants (today: MCP bearer tokens — each token row carries
+    /// a `tenant_id` and the MCP server reads them to route requests).
+    ///
+    /// The name `<db_prefix>__admin` (double underscore) is reserved —
+    /// the sanitizer never produces it for a normal tenant DB because
+    /// the natural format is `<db_prefix>_<sanitized_tenant_id>` (one
+    /// underscore) and tenant_ids would have to start with `_admin` to
+    /// collide. New tenant provisioning should reject such ids.
+    pub fn admin_db(&self) -> mongodb::Database {
+        self.client.database(&self.admin_db_name())
+    }
+
+    /// Name of the admin database — public so tests / operators can
+    /// drop it via the raw client.
+    pub fn admin_db_name(&self) -> String {
+        format!("{}__admin", self.db_prefix)
+    }
+
     /// List every Mongo database currently belonging to this pool,
     /// identified by the `<db_prefix>_` prefix. The result is the raw
     /// database names — opening one for offboarding/cleanup goes