chore: replace timeline diagram with gantt chart to remove arrows

Mermaid timeline diagrams render dashed downward arrows that can't
be hidden. Switched to gantt chart for a cleaner horizontal bar
visualization showing TLS certificate lifespan reduction from
5 years (2015) to 47 days (2029).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -17,14 +17,18 @@ TLS certificate lifespans are shrinking fast. The CA/Browser Forum passed [Ballo
 certctl is a self-hosted platform that automates the entire certificate lifecycle — from issuance through renewal to deployment — with zero human intervention. It works with any certificate authority, deploys to any server, and keeps private keys on your infrastructure where they belong.
 
 ```mermaid
-timeline
+gantt
     title TLS Certificate Maximum Lifespan (CA/Browser Forum Ballot SC-081v3)
-    2015 : 5 years
-    2018 : 825 days
-    2020 : 398 days
-    March 2026 : 200 days
-    March 2027 : 100 days
-    March 2029 : 47 days
+    dateFormat YYYY
+    axisFormat %Y
+    todayMarker off
+    section Lifespan
+        5 years (1825 days)    :done, 2015, 2018
+        825 days               :done, 2018, 2020
+        398 days               :active, 2020, 2026
+        200 days               :crit, 2026, 2027
+        100 days               :crit, 2027, 2029
+        47 days                :crit, 2029, 2031
 ```
 
 ## Documentation