Add APY rate snapshots endpoint

[Sikkra]

Resolves #30.

Summary:

• Added a D1 rate_snapshots table with pool, asset, net/interest/BLND rate components, utilization, combined BLND eps, and capture time.
• Cron now prunes snapshots older than 365 days and writes one snapshot for every fetched pool asset on each tick.
• Added public GET /rate-snapshots with window, pool_id, asset_symbol, asset_id, and bounded limit parameters.
• Documented the snapshot retention and endpoint contract in alerts/README.md.
• Added Cloudflare Worker types and cleaned up a dead helper so npx tsc --noEmit passes.

Verification:

• npm install
• npx tsc --noEmit
• git diff --check

Note: npm install reported 6 existing dev dependency audit findings in the alert package dependency tree.