feat: add private Docker registry authentication for base images

Add per-app registry credentials that are passed to Docker during image
builds, allowing apps to use base images from private registries.

- New registry_credentials table (migration 007)
- RegistryCredential model with full CRUD operations
- Docker client passes AuthConfigs to ImageBuild when credentials exist
- Deploy service fetches app registry credentials before builds
- Web UI section for managing registry credentials (add/edit/delete)
- Comprehensive unit tests for model and auth config builder
- README updated to list the feature

internal/database/migrations/007_add_registry_credentials.sql

@@ -0,0 +1,11 @@
+-- Add registry credentials for private Docker registry authentication during builds
+CREATE TABLE registry_credentials (
+    id INTEGER PRIMARY KEY,
+    app_id TEXT NOT NULL REFERENCES apps(id) ON DELETE CASCADE,
+    registry TEXT NOT NULL,
+    username TEXT NOT NULL,
+    password TEXT NOT NULL,
+    UNIQUE(app_id, registry)
+);
+
+CREATE INDEX idx_registry_credentials_app_id ON registry_credentials(app_id);