fix(edge): stop short clips waiting a minute on the NAS and make the OpenAI STT fallback usable

A five-second phone recording took over a minute: stt-proxy waited up to
60 s for the self-hosted gateway, whose GPU endpoint was off and whose NAS CPU
Whisper needs 30-90 s per clip. With a direct provider configured the gateway
now gets 5 s plus the clip length (30 s cap).

The direct OpenAI fallback never produced a result. The production key held
characters that are not valid in an HTTP header, so every request threw while
being built; provider keys are now stripped of BOM/zero-width characters and a
still-invalid key counts as not configured. whisper-1 verbose_json reports the
language by name, which the result contract rejected; names now map to codes.
Fail-closed responses list each provider's failure (status or error class,
no secrets) so an outage can be diagnosed without log access.

server/supabase/functions/llm-proxy/index.ts

@@ -28,6 +28,7 @@ import {
   type GenerationPurpose,
 } from '../_shared/generation-receipt.ts'
 import { buildAnthropicSystemBlocks } from '../_shared/generative-ai-safety.ts'
+import { readProviderKey } from '../_shared/provider-key.ts'
 
 /** 티어별 허용 모델 — free는 Haiku만, pro는 +Sonnet, pro_plus는 +Opus, team/enterprise는 전 모델 */
 const TIER_MODELS: Record<Tier, string[]> = {
@@ -96,7 +97,7 @@ Deno.serve(async (req: Request) => {
     )
 
     // A deployment without a provider must not consume quota or fabricate an answer.
-    const anthropicKey = Deno.env.get('ANTHROPIC_API_KEY')?.trim() ?? ''
+    const anthropicKey = readProviderKey('ANTHROPIC_API_KEY')
     if (!anthropicKey) {
       return new Response(JSON.stringify({ error: 'provider_unavailable' }), {
         status: 503,