Remove public internet features and fix upstream source UI (#107)

- Remove is_public field from upstream sources (all sources are internal)
- Remove allow_public_internet setting from cache settings
- Remove seeding of public registry URLs
- Fix connectivity test to not follow redirects (fixes Artifactory error)
- Add dedicated Test column with OK/Error status badges
- Auto-test sources after save
- Add error modal for viewing full error details
- Fix table layout (no-wrap on source name)
- Add ORCHARD_PURGE_SEED_DATA to stage helm values

backend/app/models.py

@@ -667,7 +667,6 @@ class UpstreamSource(Base):
     source_type = Column(String(50), default="generic", nullable=False)
     url = Column(String(2048), nullable=False)
     enabled = Column(Boolean, default=False, nullable=False)
-    is_public = Column(Boolean, default=True, nullable=False)
     auth_type = Column(String(20), default="none", nullable=False)
     username = Column(String(255))
     password_encrypted = Column(LargeBinary)
@@ -684,7 +683,6 @@ class UpstreamSource(Base):
     __table_args__ = (
         Index("idx_upstream_sources_enabled", "enabled"),
         Index("idx_upstream_sources_source_type", "source_type"),
-        Index("idx_upstream_sources_is_public", "is_public"),
         Index("idx_upstream_sources_priority", "priority"),
         CheckConstraint(
             "source_type IN ('npm', 'pypi', 'maven', 'docker', 'helm', 'nuget', 'deb', 'rpm', 'generic')",
@@ -747,13 +745,12 @@ class UpstreamSource(Base):
 class CacheSettings(Base):
     """Global cache settings (singleton table).
 
-    Controls behavior of the upstream caching system including air-gap mode.
+    Controls behavior of the upstream caching system.
     """
 
     __tablename__ = "cache_settings"
 
     id = Column(Integer, primary_key=True, default=1)
-    allow_public_internet = Column(Boolean, default=True, nullable=False)
     auto_create_system_projects = Column(Boolean, default=True, nullable=False)
     created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
     updated_at = Column(