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-778619a2d55b7e311f9d8bfd75335a1605065f136bf350b030d1b86d3cc92b37.json

@@ -0,0 +1,22 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n        SELECT EXISTS(SELECT 1 FROM tags WHERE LOWER(name) = LOWER($1)) as \"exists!\"\n        ",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "exists!",
+        "type_info": "Bool"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Text"
+      ]
+    },
+    "nullable": [
+      null
+    ]
+  },
+  "hash": "778619a2d55b7e311f9d8bfd75335a1605065f136bf350b030d1b86d3cc92b37"
+}