feat: add comprehensive tagging system with cooccurrence tracking

- Add tags, project_tags, and tag_cooccurrence tables with proper indexes
- Implement full CRUD API endpoints for tag management
- Add tag association endpoints for projects with automatic cooccurrence updates
- Include related tags and project filtering by tag functionality

.sqlx/query-a55f7d64fc0f41f150663aae82ac6fbcabf90d2627d194c3fb23f80c24dcc57a.json

@@ -0,0 +1,22 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n        SELECT EXISTS(SELECT 1 FROM tags WHERE slug = $1) as \"exists!\"\n        ",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "exists!",
+        "type_info": "Bool"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Text"
+      ]
+    },
+    "nullable": [
+      null
+    ]
+  },
+  "hash": "a55f7d64fc0f41f150663aae82ac6fbcabf90d2627d194c3fb23f80c24dcc57a"
+}