Fix infinite recrawl loop with skip-identical-content

Add last_crawled timestamp tracking to fix fetchSnapshotsFromHistory()
infinite loop when SkipIdenticalContent=true. Now tracks actual crawl
attempts separately from content changes via database DEFAULT timestamps.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

db/db_queries.go

@@ -94,7 +94,8 @@ links = :links,
 lang = :lang,
 response_code = :response_code,
 error = :error,
-header = :header
+header = :header,
+last_crawled = CURRENT_TIMESTAMP
 WHERE id = :id
 RETURNING id
 `
@@ -139,4 +140,9 @@ RETURNING id
         AND timestamp BETWEEN $2 AND $3
         ORDER BY timestamp DESC
     `
+	// New query to record crawl attempt when content is identical (no new snapshot needed)
+	SQL_RECORD_CRAWL_ATTEMPT = `
+        INSERT INTO snapshots (url, host, mimetype, response_code, error)
+        VALUES ($1, $2, $3, $4, $5)
+    `
 )