feat(03-02): add voice and image/video Tier 3 providers

- Deepgram (hex40, low confidence)
- ElevenLabs (hex32, XI_API_KEY header)
- Stability AI (sk- prefix, medium confidence)
- Runway (keyword-only)
- Midjourney (keyword-only, no official API)

Completes PROV-03: 12 Tier 3 Specialized providers (with pre-existing huggingface).

pkg/providers/definitions/stability.yaml

@@ -0,0 +1,22 @@
+format_version: 1
+name: stability
+display_name: Stability AI
+tier: 3
+last_verified: "2026-04-05"
+keywords:
+  - "stability"
+  - "STABILITY_API_KEY"
+  - "STABILITY_KEY"
+  - "api.stability.ai"
+  - "stable-diffusion"
+patterns:
+  - regex: 'sk-[A-Za-z0-9]{48}'
+    entropy_min: 4.0
+    confidence: medium
+verify:
+  method: GET
+  url: https://api.stability.ai/v1/user/account
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]