fix(ai): defer env validation to request time to prevent import crash (#263)

[Shevilll]

Fixes #263

What

Backend/app/routes/ai.py read its environment variables, ran an if not all([...]): raise ValueError(...) check, and called create_client(...) at module import time. Because app/main.py imports this module on startup (from app.routes import ai), a single missing env var (SUPABASE_URL, SUPABASE_KEY, or GEMINI_API_KEY) crashed the entire app import — taking down unrelated routes (post, chat, match) along with the AI endpoints.

This PR moves the env reads and client construction out of module scope into lazy, call-time helpers:

• get_supabase_client() — lazily builds and caches the Supabase client.
• get_gemini_api_key() — reads the Gemini key at call time.

When configuration is missing, the AI handlers now raise HTTPException(status_code=503, detail=...) with a clear message, instead of crashing import. This mirrors the convention already used by get_youtube_channel_info in the same file.

Why

Importing the AI router should never be able to take down the whole FastAPI app. With this change the app starts and all unrelated endpoints work even when AI config is absent; only the AI endpoints return a clean 503.

Verification

• python -m py_compile Backend/app/routes/ai.py passes.
• Importing the module with all env vars unset no longer raises, and create_client is not invoked at import time.
• trending_niches() and the Gemini path return a clean 503 with a descriptive message when their config is missing.
• No behavior change when the env vars are present.

Scope

Single-file, minimal change to Backend/app/routes/ai.py. No unrelated refactors.

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Reviewing files that changed from the base of the PR and between a61c801 and ec954d8.

📒 Files selected for processing (1)

• Backend/app/routes/ai.py

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[Shevilll]

Hi team! I've verified that all local checks are green and the module import crash issue is resolved. Please let me know if there's any feedback or if we are good to merge this! Thanks!