Implement cursor/keyset pagination + soft-delete scoping for Users and Tags (mirror Posts fix)

[grantfox-oss]

Module: meridian-api/src/users, meridian-api/src/tag, meridian-api/src/common

Problem
Issue #8 fixed Posts pagination, but users.controller.ts and
tag/tag.controller.ts appear to still use offset ParseIntPipe pagination with
no cursor support and no soft-delete global filter. User listing likely leaks
deactivated accounts and Tags has no uniqueness/normalization, so the same tag
("DeFi" vs "defi") fragments. There is also no N+1 guard for users' related
entities (posts, metaoptions) — common/pagination exists but isn't reused.

Deliverables

• Reuse the common/pagination keyset implementation for UsersService and
  TagsService; return { data, nextCursor, total }.
• Add a soft-delete scope so deactivated users / removed tags are excluded
  by default but restorable.
• Normalize tags (lowercase/trim + unique index) and dedupe on create.
• Add relations/join loading to kill N+1; prove with a query-count test.
• Tests for cursor pagination, soft-delete exclusion, tag normalization.

Acceptance criteria

• Cursor pagination + count assertion (no N+1) on both modules.
• Deactivated users absent from listings.
• Tag create dedupes; nest test green.

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[A-one-tech]

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[smartalee]

Hey 👋, I would like to take this please 🙌

[grantfox-oss]

🦊 GrantFox — @smartalee has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #614)
2. Your PR will be reviewed by the MERIDIAN-CITY maintainers

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