fix(tandoor): use File constructor for proper multipart uploads

- Remove unreliable URL pass-through strategy (image_url field)
- Always download and upload images as File objects
- Get MIME type from HTTP response headers for URLs
- Use File constructor (not just Blob) for proper multipart metadata
- Add comprehensive error logging with headers and file metadata
- Simplify to single reliable upload path

Fixes 400 'Upload a valid image' error caused by Blob not providing
proper filename/MIME metadata in multipart form data.

secrets/auth.json

@@ -5,7 +5,7 @@
       "value": "SDRORLyWEsWWty2ZoVGdER",
       "domain": ".instagram.com",
       "path": "/",
-      "expires": 1800849269.656302,
+      "expires": 1800850168.116943,
       "httpOnly": false,
       "secure": true,
       "sameSite": "Lax"
@@ -45,7 +45,7 @@
       "value": "59661903731",
       "domain": ".instagram.com",
       "path": "/",
-      "expires": 1774065269.656394,
+      "expires": 1774066168.117026,
       "httpOnly": false,
       "secure": true,
       "sameSite": "None"
@@ -72,7 +72,7 @@
     },
     {
       "name": "rur",
-      "value": "\"CLN\\05459661903731\\0541797825269:01fe6904bc7d85ccbfea5233062783089ac963caf6202742eb0b112bd5ab4f6ef965e2f4\"",
+      "value": "\"CLN\\05459661903731\\0541797826168:01fe0c70391c9f4322366f6ea648ca6647818cd2ca16c215419a55558746287e2865245d\"",
       "domain": ".instagram.com",
       "path": "/",
       "expires": -1,
@@ -87,7 +87,7 @@
       "localStorage": [
         {
           "name": "chatd-deviceid",
-          "value": "11f6cbef-22a3-4c0b-9558-7a83fd40e521"
+          "value": "0265e9b3-0083-498a-9424-7a08289dfb45"
         },
         {
           "name": "hb_timestamp",
@@ -95,7 +95,7 @@
         },
         {
           "name": "IGSession",
-          "value": "6m2tlb:1766291070793"
+          "value": "6m2tlb:1766291968512"
         },
         {
           "name": "pixel_fire_ts",
@@ -107,7 +107,7 @@
         },
         {
           "name": "Session",
-          "value": "0f4qnx:1766289305793"
+          "value": "nrg3hr:1766290203512"
         },
         {
           "name": "has_interop_upgraded",