Expand free tier to 400 exercises and add attribution snippets

ATTRIBUTION.md

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+# RepDB Free Tier — attribution snippets
+
+The free tier requires one visible attribution link in your app's about or
+credits screen, your project's README, or your website footer. Copy whichever
+format fits your project.
+
+## Markdown
+
+```md
+Exercise data by [RepDB](https://repdb.co/free-exercise-dataset)
+```
+
+## HTML
+
+```html
+Exercise data by <a href="https://repdb.co/free-exercise-dataset">RepDB (repdb.co)</a>
+```
+
+## Plain text / app credits
+
+```text
+Exercise data by RepDB (repdb.co)
+https://repdb.co/free-exercise-dataset
+```
+
+Keep the attribution visible to users. The full license is in `LICENSE.md`.
+For the complete growing catalog, 1024px classic images, transparent
+backgrounds, SQLite, and animations, see https://repdb.co/pricing.