Fix _meta field format for MCP spec compliance (#367)

## Summary

Updates the `_meta` field structure to be compliant with the MCP
specification by using proper `prefix/name` format for vendor
extensions.

### Changes

- **Registry metadata**: `io.modelcontextprotocol.registry` →
`io.modelcontextprotocol.registry/official`
- **Publisher metadata**: `publisher` →
`io.modelcontextprotocol.registry/publisher-provided`
- **Database migration**: Updates indexes to use new field paths
- **Updated all code**: Service layer, validators, tests, and seed data

### Format Before
```json
{
  "_meta": {
    "publisher": { "license": "MIT" },
    "io.modelcontextprotocol.registry": { "id": "...", "is_latest": true }
  }
}
```

### Format After
```json
{
  "_meta": {
    "io.modelcontextprotocol.registry/publisher-provided": { "license": "MIT" },
    "io.modelcontextprotocol.registry/official": { "id": "...", "is_latest": true }
  }
}
```

This resolves the naming collision with the planned top-level
`publisher` field and ensures compliance with MCP specification
requirements.

Fixes #356

docs/reference/server-json/server.schema.json

@@ -453,12 +453,12 @@
               "description": "Extension metadata using reverse DNS namespacing for vendor-specific data",
               "additionalProperties": true,
               "properties": {
-                "publisher": {
+                "io.modelcontextprotocol.registry/publisher-provided": {
                   "type": "object",
                   "description": "Publisher-provided metadata for downstream registries",
                   "additionalProperties": true
                 },
-                "io.modelcontextprotocol.registry": {
+                "io.modelcontextprotocol.registry/official": {
                   "type": "object",
                   "description": "Official MCP registry metadata (read-only, added by registry)",
                   "additionalProperties": true