Fix snapshot overwrite logic to preserve successful responses

- Prevent overwriting snapshots that have valid response codes
- Ensure URL is removed from queue when snapshot update is skipped
- Add last_crawled timestamp tracking for better crawl scheduling
- Remove SkipIdenticalContent flag, simplify content deduplication logic
- Update database schema with last_crawled column and indexes

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

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

misc/sql/cleanup_duplicate_snapshots.sql

@@ -0,0 +1,115 @@
+-- Cleanup script for snapshots table after adding last_crawled column
+-- This script consolidates multiple snapshots per URL by:
+-- 1. Keeping the latest snapshot with content (non-null gemtext OR data)
+-- 2. Setting its last_crawled to the most recent timestamp from any snapshot for that URL
+-- 3. Deleting all other snapshots for URLs with multiple snapshots
+--
+-- IMPORTANT: This script will permanently delete data. Make sure to backup your database first!
+
+BEGIN;
+
+-- Update last_crawled for URLs with multiple snapshots
+-- Keep the latest snapshot with content and update its last_crawled to the most recent timestamp
+WITH url_snapshots AS (
+    -- Get all snapshots grouped by URL with row numbers
+    SELECT 
+        id,
+        url,
+        timestamp,
+        last_crawled,
+        gemtext,
+        data,
+        ROW_NUMBER() OVER (PARTITION BY url ORDER BY timestamp DESC) as rn_by_timestamp
+    FROM snapshots
+),
+latest_content_snapshots AS (
+    -- Find the latest snapshot with content for each URL
+    SELECT 
+        url,
+        id as keep_id,
+        timestamp as keep_timestamp
+    FROM url_snapshots
+    WHERE (gemtext IS NOT NULL OR data IS NOT NULL)
+        AND rn_by_timestamp = (
+            SELECT MIN(rn_by_timestamp) 
+            FROM url_snapshots us2 
+            WHERE us2.url = url_snapshots.url 
+                AND (us2.gemtext IS NOT NULL OR us2.data IS NOT NULL)
+        )
+),
+most_recent_timestamps AS (
+    -- Get the most recent timestamp (last_crawled or timestamp) for each URL
+    SELECT 
+        url,
+        GREATEST(
+            MAX(timestamp),
+            COALESCE(MAX(last_crawled), '1970-01-01'::timestamp)
+        ) as most_recent_time
+    FROM snapshots
+    GROUP BY url
+)
+-- Update the last_crawled of snapshots we're keeping
+UPDATE snapshots 
+SET last_crawled = mrt.most_recent_time
+FROM latest_content_snapshots lcs
+JOIN most_recent_timestamps mrt ON lcs.url = mrt.url
+WHERE snapshots.id = lcs.keep_id;
+
+-- Delete all other snapshots for URLs that have multiple snapshots
+WITH url_snapshots AS (
+    SELECT 
+        id,
+        url,
+        timestamp,
+        gemtext,
+        data,
+        ROW_NUMBER() OVER (PARTITION BY url ORDER BY timestamp DESC) as rn_by_timestamp
+    FROM snapshots
+),
+latest_content_snapshots AS (
+    -- Find the latest snapshot with content for each URL
+    SELECT 
+        url,
+        id as keep_id
+    FROM url_snapshots
+    WHERE (gemtext IS NOT NULL OR data IS NOT NULL)
+        AND rn_by_timestamp = (
+            SELECT MIN(rn_by_timestamp) 
+            FROM url_snapshots us2 
+            WHERE us2.url = url_snapshots.url 
+                AND (us2.gemtext IS NOT NULL OR us2.data IS NOT NULL)
+        )
+),
+snapshots_to_delete AS (
+    -- Find snapshots to delete (all except the ones we're keeping)
+    SELECT s.id
+    FROM snapshots s
+    LEFT JOIN latest_content_snapshots lcs ON s.id = lcs.keep_id
+    WHERE lcs.keep_id IS NULL
+        AND s.url IN (
+            -- Only for URLs that have multiple snapshots
+            SELECT url 
+            FROM snapshots 
+            GROUP BY url 
+            HAVING COUNT(*) > 1
+        )
+)
+DELETE FROM snapshots 
+WHERE id IN (SELECT id FROM snapshots_to_delete);
+
+-- Show summary of changes
+SELECT 
+    'Cleanup completed. Remaining snapshots: ' || COUNT(*) as summary
+FROM snapshots;
+
+-- Show URLs that still have multiple snapshots (should be 0 after cleanup)
+SELECT 
+    'URLs with multiple snapshots after cleanup: ' || COUNT(*) as validation
+FROM (
+    SELECT url 
+    FROM snapshots 
+    GROUP BY url 
+    HAVING COUNT(*) > 1
+) multi_snapshots;
+
+COMMIT;