feat(access): Add HTTP Basic Authentication for domains

[nfebe]

Add HTTP Basic Authentication as a visitor access mode for domains and paths. Operators configure credentials in FlatRun while Nginx presents the browser's native username and password prompt.

Agents use separate service credentials scoped to approved domains and paths. FlatRun injects the Basic Authorization header only for granted requests and keeps the password out of prompts, logs, and tool output.

Constraint: Basic Authentication can only be enabled for domains served over HTTPS.

Checklist

• Add Basic Authentication as a visitor access mode
• Store browser password hashes and never return passwords through the API
• Configure Nginx to protect the selected domain and path
• Add username and password fields to the domain form
• Add separate service credentials for agent access
• Scope each service credential to approved domains and paths
• Grant agents access to selected service credentials
• Inject credentials inside the HTTP request executor without exposing passwords
• Cover browser and agent access, denied scopes, and secret redaction through HTTP tests