wiki: add 4 new articles from archive, merge 8 archive notes into existing articles (73 articles)

New: mdadm RAID rebuild, Mastodon instance tuning, Ventoy, Fedora networking/kernel recovery.
Merged: Glacier Deep Archive into rsync, SpamAssassin into hardening checklist,
OBS captions/VLC capture into OBS setup, yt-dlp subtitles/temp fix into yt-dlp.
Updated index.md, README.md, SUMMARY.md with 21 previously missing articles.
Fixed merge conflict in index.md Recently Updated table.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

02-selfhosting/security/linux-server-hardening-checklist.md

@@ -194,6 +194,38 @@ sudo systemctl disable --now servicename
 
 Common ones to disable on a dedicated server: `avahi-daemon`, `cups`, `bluetooth`.
 
+## 8. Mail Server: SpamAssassin
+
+If you're running Postfix (like on majormail), SpamAssassin filters incoming spam before it hits your mailbox.
+
+**Install (Fedora/RHEL):**
+
+```bash
+sudo dnf install spamassassin
+sudo systemctl enable --now spamassassin
+```
+
+**Integrate with Postfix** by adding a content filter in `/etc/postfix/master.cf`. See the [full setup guide](https://www.davekb.com/browse_computer_tips:spamassassin_with_postfix:txt) for Postfix integration on RedHat-based systems.
+
+**Train the filter with sa-learn:**
+
+SpamAssassin gets better when you feed it examples of spam and ham (legitimate mail):
+
+```bash
+# Train on known spam
+sa-learn --spam /path/to/spam-folder/
+
+# Train on known good mail
+sa-learn --ham /path/to/ham-folder/
+
+# Check what sa-learn knows
+sa-learn --dump magic
+```
+
+Run `sa-learn` periodically against your Maildir to keep the Bayesian filter accurate. The more examples it sees, the fewer false positives and missed spam you'll get.
+
+Reference: [sa-learn documentation](https://spamassassin.apache.org/full/3.0.x/dist/doc/sa-learn.html)
+
 ## Gotchas & Notes
 
 - **Don't lock yourself out.** Test SSH key auth in a second terminal before disabling passwords. Keep the original session open.