fix: replace dead YouTube APIs with yt-dlp subprocess; fix all JSON panic crashes; add yt-dlp helper functions

- YouTube MP3/MP4: replaced dead savetube.media + ytdlpyton.nvlgroup.my.id with yt-dlp subprocess
- Added find_ytdlp() + run_ytdlp_json() helpers with spawn_blocking for >64KB stdout
- Fixed all resp['key'] -> resp.get('key') panics in Twitter, Spotify, KrakenFiles, Danbooru
- Removed dead fetch_youtube_v2_mp3/mp4 + decrypt_savetube functions
- ScraperError::Http -> AppError::ScraperError -> HTTP 502 instead of 500
- All endpoints now return graceful 502 with error message instead of crashing 500

Tested with real URLs:
- YouTube MP3: 200, success, real audio URL extracted
- YouTube MP4: 200, success, 48 format URLs extracted
- TikTok: 200 (tikwm provider)
- MediaFire: 200 (success)

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1787964612,
+        "narHash": "sha256-0N9nghg3nwzX6b6qc77EzjR9cu/Z+UR66FlfsCqiURs=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e8be7818e19ada32105a8af937a6a473b38167ca",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}