perf: eliminate redundant D1 settings queries per API request

[Copilot]

getSettings() was being called twice per request — once in requireApiAuth middleware and again in each route handler — and each call issued 6 sequential D1 round trips.

Changes

src/settings.ts — batch reads and writes

• getSettings: replace 6× dbFirst with single dbAll using WHERE key IN (...) + Map lookup — 6 D1 round trips → 1
• saveSettings: replace 6× dbRun with env.DB.batch() — 6 D1 writes → 1

src/auth.ts + src/routes/openai.ts — cache settings in request context

Store settings in Hono context after the single fetch in requireApiAuth; route handlers read from context instead of re-querying.

// auth.ts — fetch once, store in context
const settings = await getSettings(c.env);
c.set("settingsBundle", settings);

// openai.ts — read from context, no DB hit
const settingsBundle = c.get("settingsBundle");

Net effect: settings cost per API request drops from 12 D1 queries to 1.

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