Fix digest unsubscribe API URL fallback

[DeFex-lab]

Overview

Fixes digest unsubscribe links so they no longer silently fall back to http://localhost:3000/api when API_URL is unset.

Related Issue

Closes #1090

Changes

• Removed the default localhost fallback from EmailDigestService.sendDigestForUser.
• Added an explicit configuration failure when API_URL is missing.
• Added email digest tests for configured unsubscribe URLs and missing API_URL.

Verification Results

• npm ci - passed
• npm test -- src/email-digest/email-digest.service.spec.ts --runInBand - passed
• node_modules\.bin\jest.cmd src/email-digest/email-digest.service.spec.ts --runInBand - passed
• node_modules\.bin\eslint.cmd src\email-digest\email-digest.service.ts src\email-digest\email-digest.service.spec.ts - passed
• git diff --check - passed
• npm test -- --runInBand - timed out after 5 minutes with no usable output
• node_modules\.bin\nest.cmd build - failed on existing unrelated TypeScript errors outside this change, including src/sessions/sessions.service.ts and src/versioning/deprecation-warning.interceptor.spec.ts

Acceptance Criteria

Criteria	Status
Digest unsubscribe links never contain localhost unless explicitly configured	Met
Missing API_URL no longer creates a dead localhost URL	Met
Configured digest link points at the configured API host	Met

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