Add standard MIT LICENSE (curation only; original data retains provider terms)

LICENSE

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+MIT License
+
+Copyright (c) 2026 SMILE Project and contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
+Notice on third-party data
+
+This repository packages and/or indexes data derived from third-party public
+datasets. Those underlying datasets are distributed under their own licenses
+and usage terms set by the original providers, which are NOT superseded by
+this MIT License. Users are responsible for reviewing and complying with
+those original terms, including any citation, attribution, non-commercial, or
+redistribution restrictions. The MIT License above applies to the curation,
+packaging, code, and documentation contributed in this repository, not to the
+underlying third-party imaging data.