feat(plex): P0 admin Config UI — connection test + library picker

- ConfigPanel 'plex' group auto-renders; adds a Test-connection button + a
  library-picker (checked sections write plex_libraries, applied on Save)
- api.testPlex + PlexTestResult/PlexSection types
- i18n config namespace: plex group + 7 fields + test strings (en/hu/de);
  also fills the previously-missing 'downloads' group label

frontend/src/i18n/locales/en/config.json

@@ -10,7 +10,9 @@
     "backfill": "Backfill",
     "shorts": "Shorts probe",
     "batch": "Batch sizes",
-    "explore": "Channel explore"
+    "explore": "Channel explore",
+    "downloads": "Downloads",
+    "plex": "Plex"
   },
   "fields": {
     "smtp_host": {
@@ -100,6 +102,34 @@
     "search_grace_days": {
       "label": "Search results — grace period (days)",
       "hint": "How long an un-kept live-search result video (nobody watched / saved / subscribed to) is kept before the discovery-cleanup job removes it."
+    },
+    "plex_enabled": {
+      "label": "Enable Plex",
+      "hint": "Expose the Plex library as a feed source with a rich player. Requires the Plex server to be reachable from this host and its media on a local mount (see the path map)."
+    },
+    "plex_server_url": {
+      "label": "Plex server URL",
+      "hint": "Base URL reachable from this host, e.g. http://192.168.1.112:32400."
+    },
+    "plex_server_token": {
+      "label": "Plex server token",
+      "hint": "The server admin X-Plex-Token. Used server-side only (metadata + images); never sent to the browser. Stored encrypted; write-only."
+    },
+    "plex_path_map": {
+      "label": "Media path map",
+      "hint": "Maps Plex's on-disk paths to this host's local mount so Siftlode can play the physical file. Newline/comma-separated plexPrefix=localPrefix pairs, e.g. /data=/plex-media. Empty = identical on both sides."
+    },
+    "plex_libraries": {
+      "label": "Exposed libraries",
+      "hint": "Comma-separated Plex section keys to expose. Empty = all sections. Use the library picker below after testing the connection."
+    },
+    "plex_sync_interval_min": {
+      "label": "Sync interval (minutes)",
+      "hint": "How often the catalog sync job mirrors Plex metadata."
+    },
+    "plex_max_transcodes": {
+      "label": "Max concurrent transcodes",
+      "hint": "Cap on simultaneous CPU transcodes for incompatible files (a later fallback). Direct-playable files aren't limited."
     }
   },
   "save": "Save",
@@ -121,5 +151,15 @@
   "testEmail": "Send test email",
   "testEmailHint": "Send a test email to your own address using the settings above, to confirm they work.",
   "testSent": "Test email sent to {{to}}",
-  "testFailed": "Test email failed — check the settings"
+  "testFailed": "Test email failed — check the settings",
+  "plexTest": "Test connection",
+  "plexTestHint": "Verify the Plex server URL + token and load the library list. Save any edits first.",
+  "plexTestDirty": "Save your changes before testing the connection.",
+  "plexTestOk": "Connected to {{name}}",
+  "plexTestFailed": "Plex connection failed — check the URL and token",
+  "plexConnected": "Connected to {{name}} {{version}}",
+  "plexPickLibraries": "Libraries to expose:",
+  "plexPickHint": "Unchecking all is the same as exposing every library. Remember to Save.",
+  "plexMovies": "Movies",
+  "plexShows": "TV Shows"
 }