Raise new-code coverage on CRM connectors + server routes to the 80% gate

[lgutschow]

The v0.9.0 PR (#5) landed with the SonarCloud new-code coverage gate red at
63% (bar is 80%). Every other gate condition is green, including all
security conditions (rating A, hotspots 100% reviewed, zero vulnerabilities).

The gap is integration/I/O code, not the image-paste feature (which is
unit-covered + browser-verified). Under-covered new code:

• connectors/email.ts (IMAP/SMTP), connectors/twitter.ts, connectors/linkedin.ts (~26-46%)
• tools/mcp-internal/social.ts, email.ts tool wrappers
• admin/server.ts routes (19%), mcp-server.ts (22%)

These need external-service mocking (IMAP/SMTP/Twitter/LinkedIn) to unit-test.

Goal: add mocked unit tests for the connector send/read paths + the admin
routes until new_coverage clears 80% on a subsequent PR.

Note: src/admin/app.js is intentionally excluded from coverage (browser DOM
code, verified in a real browser) — see sonar-project.properties.