Optimize scan process with quick/deep modes and archive tag merging

Quick scan (default):
- Pre-load all ImportTask records for O(1) duplicate checks
- Batch task creation commits (50 at a time)
- Cache import settings (5-min TTL)
- Skip pHash comparison for speed

Deep scan (on-demand):
- Full reprocessing of all files
- pHash similarity detection
- Optional thumbnail regeneration
- Optional sidecar re-application

Archive tag merging:
- Add archive tags to existing images when duplicates found
- Works for both hash and pHash duplicate detection

Also fixes:
- Add missing source/post tag icons to gallery templates

app/templates/_gallery_item.html

@@ -26,6 +26,10 @@
             🎭 {{ t.name.split(':', 1)[1] if ':' in t.name else t.name }}
           {% elif t.kind == 'rating' %}
             ⚠️ {{ t.name.split(':', 1)[1] if ':' in t.name else t.name }}
+          {% elif t.kind == 'source' %}
+            🌐 {{ t.name.split(':', 1)[1] if ':' in t.name else t.name }}
+          {% elif t.kind == 'post' %}
+            📌 {{ t.name.split(':', 1)[1] if ':' in t.name else t.name }}
           {% else %}
             #{{ t.name }}
           {% endif %}