feat: add archiving of platform media files (#1)

* chore: specify non-ignored `.md` files

* refactor: rename youtube downloader to ytdlp

More generic name since yt-dlp supports many sites beyond YouTube.

* feat: add local file downloader

Supports file:// URLs for archiving local files.

* deps: add regex crate for URL pattern matching

* feat: expand source detection with granular YouTube types

- Split Source::YouTube into YouTubeVideo, YouTubePlaylist, YouTubeChannel
- Add Source::X for Twitter/X posts
- Add Source::Local for file:// URLs
- Add regex-based URL pattern matching for YouTube URLs
- Add shorthand schemes (yt:video/ID, youtube:playlist/ID, etc.)
- Add comprehensive tests for all URL patterns

* docs: update README milestones

Mark YouTube videos, Twitter videos, and local files as done.

* chore: update flake.lock

* feat: add shorthand schemes for X/Twitter media

* chore: move docs into docs dir

* Remove temp file using timestamp path

Delete the temp entry at store_path/temp/<timestamp> in both
the hash-exists and success paths. Stop constructing the full filename
with extension and remove the early process::exit to de-duplicate
cleanup.

* Add Nix caches and default flake package

* Add social platform source detection and update milestones

* Tighten social URL matching to avoid false positives

* Mark media archiving milestone complete

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1760284886,
-        "narHash": "sha256-TK9Kr0BYBQ/1P5kAsnNQhmWWKgmZXwUQr4ZMjCzWf2c=",
+        "lastModified": 1761672384,
+        "narHash": "sha256-o9KF3DJL7g7iYMZq9SWgfS1BFlNbsm6xplRjVlOCkXI=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "cf3f5c4def3c7b5f1fc012b3d839575dbe552d43",
+        "rev": "08dacfca559e1d7da38f3cf05f1f45ee9bfd213c",
         "type": "github"
       },
       "original": {