feat: merge ThreeTxtFileHelper into URLNotesGrabberCORE

Folds the standalone ThreeTxtFileHelper tool into URLNotesGrabberCORE so
text-file ingest/output/correct lives alongside the API scraper. Adds
new flags -ingest, -output, -correct (with -apply), -updatepaths, and a
one-time -importposts <posts.db> migration.

Schema: Blogs.TTFolderPath and Posts.PostType are added by an idempotent
migration. On (BlogName, PostID) collisions, content columns are
overwritten while engagement columns (ByLikes, RootBlogName, RootURL,
HasNotesGathered, NotFound, NotesGatheredDateTime, Likes*) are preserved.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

URLNotesGrabberCORE/appsettings.json

@@ -12,7 +12,12 @@
     "PostIDToExclude": "730084470076686336,726309940037287936,639974102120235008,184045126218",
     "EnableFileLogging": false,
     "LogTraversalRecordImports": false,
-    "LikesRefreshCooldownDays": 7
+    "LikesRefreshCooldownDays": 7,
+    "PathTTRoot": "",
+    "PathTTBackup": "",
+    "PathPrefixes": "prefixes.txt",
+    "PathCorrectionReport": "correction_report.txt",
+    "PathCorrectionApplied": "correction_applied.txt"
   },
   "TumblrApi": {
     "ConsumerKey": "PtsBCGumcsgihyynxUh8b47Jfmi7uXhEIOU46bmhdsUSJ5mEP3",