Use APP_BASE_URL for all OAuth redirects

[eswan18]

Summary

• Follow-up to Fix OAuth redirect URI behind reverse proxy #115 — the redirect_uri was fixed but all NextResponse.redirect calls still used request.url as the base URL, which resolves to 0.0.0.0:3000 inside the pod
• Define baseUrl = process.env.APP_BASE_URL ?? request.nextUrl.origin at the top of each route handler and use it everywhere
• Falls back to request origin for local development when APP_BASE_URL isn't set

Test plan

• Deploy to staging, verify full OAuth login flow completes and redirects to forecasting-staging.tailc06f30.ts.net
• Verify error redirects also go to the correct host
• Verify local dev still works without APP_BASE_URL set

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