test: add unit tests for TagsService, TagsController, and slugify (#202)

[iyanumajekodunmi756]

• api/src/tags/slugify.spec.ts: 12 cases covering NFD normalisation, diacritics, special chars, unicode, length cap, and non-string input
• api/src/tags/tags.service.spec.ts: 11 cases covering list pagination (hasMore true/false), attachToStream (new tag, reuse existing slug, whitespace trimming, empty/non-alphanumeric name), and detachFromStream (success, tag not found, tag not attached to stream)
• api/src/tags/tags.controller.spec.ts: 8 cases covering TagsListController defaults and delegation, StreamTagsController guard reflection, attach delegation, and detach delegation

All 83 tests pass (31 new + 52 existing) across 9 suites.

Closes #202

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closes #202

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