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

internal/database/migrations/004_update_meta_field_format.sql

@@ -0,0 +1,11 @@
+-- Update database indexes to use new MCP spec compliant _meta field paths
+-- Data has been updated manually, only need to update indexes
+
+-- Drop old indexes that reference the old paths
+DROP INDEX IF EXISTS idx_servers_name_latest;
+DROP INDEX IF EXISTS idx_servers_updated_at;
+
+-- Create new indexes with updated paths
+CREATE INDEX idx_servers_name_latest ON servers ((value->>'name'), (value->'_meta'->'io.modelcontextprotocol.registry/official'->>'is_latest'))
+    WHERE (value->'_meta'->'io.modelcontextprotocol.registry/official'->>'is_latest')::boolean = true;
+CREATE INDEX idx_servers_updated_at ON servers ((value->'_meta'->'io.modelcontextprotocol.registry/official'->>'updated_at'));