Add URL shortener for link tracking and shorter posts

- New short_urls and clicks tables for URL mapping and analytics
- /r/{code} redirect endpoint with click tracking
- Short URLs use 6-char base64 hash codes (26 chars total)
- Publish loop now shortens article links and enclosure URLs
- Enables podcast audio URLs to fit in posts (139 → 26 chars)
- Tracks: timestamp, referrer, user agent, anonymized IP

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crawler.go

@@ -246,8 +246,25 @@ func (c *Crawler) StartPublishLoop() {
 				sessions[account] = session
 			}
 
+			// Shorten URLs before publishing
+			itemToPublish := item
+			if item.Link != "" {
+				if shortURL, err := c.GetShortURLForPost(item.Link, &item.ID, item.FeedURL); err == nil {
+					itemToPublish.Link = shortURL
+				}
+			}
+			if item.Enclosure != nil && item.Enclosure.URL != "" {
+				if shortURL, err := c.GetShortURLForPost(item.Enclosure.URL, &item.ID, item.FeedURL); err == nil {
+					itemToPublish.Enclosure = &Enclosure{
+						URL:    shortURL,
+						Type:   item.Enclosure.Type,
+						Length: item.Enclosure.Length,
+					}
+				}
+			}
+
 			// Publish the item
-			uri, err := publisher.PublishItem(session, &item)
+			uri, err := publisher.PublishItem(session, &itemToPublish)
 			if err != nil {
 				fmt.Printf("Publish: failed item %d: %v\n", item.ID, err)
 				// Clear session cache on auth errors