feat: add OAuth for hosted MCP connectors

[kapelame]

Summary

Add OAuth support for hosted MCP connectors on top of the MCP Connector Directory core in #223. Notion is introduced directly with the final shared mcp_oauth credential model, so imported capabilities never depend on the legacy notion_integration token kind.

Scope

• Discover OAuth protected-resource and authorization-server metadata.
• Support dynamic client registration and PKCE authorization in a popup flow.
• Add the built-in mcp_oauth credential kind and migration for existing deployments.
• Store and refresh OAuth tokens through the existing encrypted secrets infrastructure.
• Store the connector identity in the existing Secret provider field and the shared kind in existing Secret metadata.
• Add Notion 1.0.0 through its official hosted MCP endpoint with the explicit default scope.
• Render authenticated Streamable HTTP MCP configuration for Claude Code, Codex, and OpenCode.
• Require a successful OAuth connection before importing an authenticated directory item.

Reuse and boundaries

• Reuses the existing secrets table, encryption service, credential kind registry, Capability import, versioning, rendering, and runtime paths.
• Adds no Connector, OAuth-token, or credential-binding table.
• Does not add capability-level credential selection; that is handled by feat: bind shared credentials to agent capabilities #230.
• Workspace Members may import an already connected connector, but only owners and admins may create or overwrite the shared Workspace OAuth credential.

Security

• OAuth endpoints come from the server-owned catalog and discovery documents, not request parameters.
• Catalog entries declare the exact requested OAuth scopes; Parsar no longer requests every scope advertised by discovery.
• PKCE S256, state validation, callback cookies, HTTP timeouts, response size limits, and issuer validation are enforced.
• The production OAuth HTTP client limits redirects, rejects redirects to HTTP, and blocks localhost, private, link-local, metadata, and DNS-resolved private destinations.
• Tokens are never returned by directory APIs or written to logs.

Verification

• make check
• Migration and Store integration smoke test
• MCP OAuth discovery, scope, redirect, private-address, exchange, and refresh tests
• Directory role, import, renderer, runtime, Store, and frontend checks
• OpenAPI regeneration
• git diff --check

Dependencies

• Stacked on feat: add curated MCP Connector Directory core #223.
• feat: bind shared credentials to agent capabilities #230 adds capability-level shared credential binding on top of this PR.