Rebrand: Rename "MCP Gateway Registry" to "AI Gateway & Registry" across codebase

## Problem Statement

The solution has evolved significantly beyond its original scope of managing Model Context Protocol (MCP) servers. It now supports:

- **A2A Agents** - Google's Agent-to-Agent protocol
- **Agent Skills** - Reusable agent capabilities
- **Virtual MCP Servers** - Dynamically composed server endpoints
- **Federation** - Cross-registry discovery and synchronization
- **Security Scanning** - YARA-based and LLM-based threat analysis
- **Semantic Search** - AI-powered discovery across all asset types

The current name "MCP Gateway Registry" no longer accurately describes the platform's capabilities. The name creates a false impression that this is only an MCP-specific tool, when in reality it is a unified AI asset management platform.

## Proposed Solution

Rename the solution to **"AI Gateway & Registry"** across all user-facing surfaces:

- Frontend UI (headers, titles, labels, meta tags, browser tab)
- Backend API (OpenAPI title, health checks, startup/shutdown logs)
- Documentation (README with prominent rename notice, mkdocs, Helm chart READMEs)
- Configuration exports (Terraform, env, YAML headers)
- CLI tool descriptions
- Well-known discovery endpoints
- Grafana dashboard titles
- Helm chart descriptions
- Terraform resource tags

### Keycloak Realm: Auto-Detection Strategy

For the Keycloak realm name (`mcp-gateway`), implement an auto-detection approach:

- **Existing installs**: Detect the legacy `mcp-gateway` realm exists and keep it untouched (no auth disruption)
- **New installs**: Default to `ai-gateway` realm via `.env.example` and `init-keycloak.sh` detection logic
- Python/Docker Compose fallback defaults stay `mcp-gateway` as a safety net for upgrades

### What NOT to Rename

- Git repository name (`mcp-gateway-registry`) - separate decision
- Docker Compose service names - would break existing deployments
- DockerHub / ECR image names - separate migration
- Internal Python package name (`mcp-registry`) - would break imports
- MCP protocol-specific references (these correctly describe the protocol)
- Terraform `var.name` prefix - would destroy/recreate all AWS resources
- Helm chart package names - would break `helm install` commands
- Asset-type labels ("MCP Servers", "Virtual MCP Servers") - these describe the asset type, not the product

## Scope

~74 string changes across ~52 files + init script detection logic, organized into 11 categories:

| Category | Files | Changes |
|----------|-------|---------|
| Frontend UI (tab, navbar, login, alt text) | 5 | ~6 |
| Backend API (FastAPI title, health, logs, well-known, config exports) | 6 | ~16 |
| CLI & Management Tools | 5 | ~7 |
| Module Docstrings | 7 | ~7 |
| Documentation (README, mkdocs, .env.example) | 6 | ~8 |
| Build Scripts | 1 | ~2 |
| Keycloak Setup Scripts (product name only) | 3 | ~8 |
| Grafana Dashboards | 3 | ~4 |
| CI/CD Workflow Files | TBD | TBD |
| Terraform (comments, tags, tfvars) | 4 | ~6 |
| Helm Charts (descriptions, READMEs, templates) | 8 | ~14 |
| Keycloak Auto-Detection Logic | 3 | New logic |

## README Rename Notice

Add a prominent notice block at the top of the README:

> **Formerly "MCP Gateway & Registry"** -- As of v1.1, this project has been renamed to **AI Gateway & Registry** to reflect its evolution beyond MCP. The platform now manages MCP Servers, A2A Agents, Agent Skills, Virtual Servers, and more AI assets, making it a truly unified AI asset management platform. The repository name remains unchanged for backward compatibility.

## User Stories

- As a field team member, I want the product name to reflect its full capabilities so that customers understand its value beyond MCP
- As a developer evaluating the platform, I want the branding to accurately represent the breadth of supported AI protocols and assets
- As an existing user, I want a smooth transition with a clear explanation of why the name changed

## Acceptance Criteria

- [ ] All user-visible UI text updated to "AI Gateway & Registry"
- [ ] Browser tab title updated
- [ ] Login page branding updated
- [ ] OpenAPI/Swagger title updated to "AI Gateway & Registry"
- [ ] Health check endpoint returns `"service": "ai-gateway-registry"`
- [ ] README has prominent rename notice explaining rationale
- [ ] Well-known discovery endpoints return new names
- [ ] CLI tool descriptions updated
- [ ] Startup/shutdown log messages updated
- [ ] Configuration export headers updated
- [ ] mkdocs site name updated
- [ ] Helm chart descriptions updated
- [ ] Terraform resource tags updated
- [ ] Grafana dashboard titles updated
- [ ] Keycloak init script auto-detects existing vs new install
- [ ] `.env.example` defaults to `ai-gateway` realm for new installs
- [ ] All existing tests pass (700+ tests, zero regressions)