fix(ai): require auth, rate limit and credits on AI endpoints (#69)

[Rudra-clrscr]

Closes #69

Problem

Five paid AI-generation routes called Gemini / Groq / Tavily before
verifying authentication (or without any auth at all). Anyone without an
account could repeatedly hit them and drain the app's API quota. The existing
rateLimit() and credit helpers were defined but never used.

Fix

Added a shared guard lib/ai-guard.ts → requireAiAccess() that runs
before any external API call and, in order:

1. 401 if the request is unauthenticated (getUser())
2. 429 if the per-user burst limit is exceeded (existing rateLimit())
3. 402 if the user is out of credits (existing consumeCredit(), ai_chat)

On success it returns the authenticated user; on failure it returns a
ready-to-send NextResponse, so each route just does:

const guard = await requireAiAccess()
if (guard.error) return guard.error
const { user } = guard

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