feat: introduce API v2 contract

[SamoTech]

Phase 12 — API v2

• Adds stable /api/v2 discovery endpoint.
• Adds v2 health, security, dependencies, and PR resources.
• Standardizes success/error envelopes with API version and resource metadata.
• Reuses existing production engines and rate limits instead of duplicating logic.
• Keeps all v1 endpoints unchanged for compatibility.
• Documents the v2 contract.

Validation required before merge.

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[vercel]

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