feat(api/v2): ✨ Implement socials routes

[kyanvde]

Closes #60.

Adds the /socials section to api-v2, covering every route the issue lists.

Route	Auth	Notes
GET /socials	authenticated team	Returns the token holder's own links
GET /:teamId/socials	public	?slug=true resolves the team by slug
POST /socials	team
PUT /socials	team	Bulk upsert
PUT /socials/:id	team
DELETE /socials/:id	team

Every route is registered both bare and behind a :teamId prefix, and the prefix is only ever allowed to name the team the token already belongs to (@TeamScope).

Notes

• The section mirrors applications/questions, which has the same shape — a team-owned resource with a bulk upsert — so the service, DTOs and tests follow it deliberately closely.
• Read auth matches v1: GET /buildteams/:id/socials was public there, and the links are rendered on the public team page, so the prefixed form is public (@OptionalAuth) while the bare form falls back to the authenticated team and answers 401 without a token.
• The bulk upsert leaves links that are not in the payload alone, rather than treating the payload as the team's full set. Deleting is what DELETE /socials/:id is for, and a silent delete-on-omit is easy to trigger by accident.
• An ID that belongs to another team answers 404, not 403 — a 403 would confirm the ID exists. Same reasoning as the questions section.
• url is validated as a non-empty string rather than with @IsUrl(). Existing rows written by v1 hold arbitrary strings (mailto: addresses, app-specific schemes), so a stricter rule would reject data the API already serves.
• Bulk payloads are capped at 100 entries, since the whole batch runs in one transaction.
• sortBy allows id, name, icon, url; filtering allows name, icon, url.

Testing

yarn ws api-v2 test — 27 suites, 155 tests, all passing. 38 of those are new:

• socials.service.spec.ts — pagination/sorting/filtering, slug resolution, team scoping on update and delete, and the upsert paths (update existing, create new, create with a caller-supplied ID, cross-team refusal, bulk limit).
• socials.controller.spec.ts — team resolution, the slug flag not leaking into the filter, and the 401 on the unprefixed listing.
• socials.routes.spec.ts — end to end through the real router, including that PUT /socials reaches the bulk upsert while PUT /socials/:id reaches the single update.

yarn ws api-v2 build passes. lint reports the 6 pre-existing unbound-method errors documented in CLAUDE.md and nothing new.

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