Migrate AI Hint feed generation

Bring dynamic hint generation into the account service while preserving the legacy key.osglab.com deployment for existing clients.

docs/HINT_FEED_MIGRATION.md

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+# AI Hint Feed migration and coexistence
+
+## Boundary
+
+`key.osglab.com` remains the legacy AI Hint Feed for installed clients that still
+use that origin. This repository must not:
+
+- change the `key.osglab.com` DNS record;
+- redirect `key.osglab.com` to `account.osglab.com`;
+- reuse or delete the legacy container, image, settings file, or data volume;
+- require the legacy service to call this account service.
+
+The migrated generator runs independently inside OSGAccountServer and publishes:
+
+- `https://account.osglab.com/v1/content/hints/manifest`
+- `https://account.osglab.com/v1/content/hints/{locale}`
+- `https://account.osglab.com/hints/manifest.json`
+- `https://account.osglab.com/hints/hints-{locale}.json`
+
+## Safe rollout
+
+1. Back up the legacy `settings.json`, `manifest.json`, `hints-zh.json`, and
+   `hints-en.json` from its persistent volume.
+2. Deploy OSGAccountServer with `HINT_FEED_ENABLED=false`.
+3. Apply Flyway migration `V24__hint_feed_generation.sql` and the matching
+   runtime grants.
+4. Use the protected admin console to review generation settings and run one
+   manual generation.
+5. Verify both v1 and legacy paths on `account.osglab.com`, including ETag/304.
+6. Set `HINT_FEED_ENABLED=true` only after the generated packs are accepted.
+7. Point only new client releases at `account.osglab.com`. Existing clients may
+   continue to use `key.osglab.com`.
+
+## Rollback
+
+Disable `HINT_FEED_ENABLED` to stop scheduled generation. Published packs remain
+available from MySQL and manual publishing remains available. No rollback step
+depends on or modifies `key.osglab.com`.
+
+Provider credentials such as `TOPHUB_API_KEY` stay in environment-backed secret
+storage. They are never written to the generation settings table or returned to
+the admin browser.