Feature: Add tag/source browser for configured connections

[smithaldon1]

Summary

Closes #217.

• Adds a read-only /tag-source-browser Protocol Operations page for configured OPC-UA nodes, MQTT topics, and BACnet objects.
• Groups source entries by protocol and connection, with filters for protocol, mapping state, mapped area/asset, quality/status, and search text.
• Links mapped source rows to normalized FactoryEvent API context when matching event data is available, without exposing endpoints, secrets, certificates, or write controls.

Test Evidence

• cd apps/web && npm test -> 18 passed
• cd apps/web && npm run lint -> passed
• cd apps/web && npm run typecheck -> passed
• cd apps/web && npm run build -> passed
• cd apps/web && PLAYWRIGHT_API_PORT=8001 PLAYWRIGHT_WEB_PORT=3001 PLAYWRIGHT_BASE_URL=http://127.0.0.1:3001 npm run test:e2e -> first sandbox run failed to bind 127.0.0.1:8001; reran with approved local port access -> 1 passed
• make test -> 266 passed
• make lint -> passed
• make typecheck -> passed
• make docs -> passed; docs are Markdown-only and no docs checker is configured yet
• git diff --check -> passed

Docs Impact

• Updated apps/web/README.md with the new route and read-only behavior.
• Updated docs/LEARNING_LOG.md with what changed, how the derived source browser works, and the next learning step.

Follow-up Work

• Replace derived mapping state with explicit mapping records when the dedicated mapping API lands.
• Add richer invalid/stale reason metadata once connector diagnostics return per-source mapping validation details.

[smithaldon1]

Reviewed and approved for merge