Configuration CRUD Framework (DB-Backed Settings & Admin REST API)

### **Proposed Change:**
Move configuration defaults from static .conf files into a _site_settings_ database table, and expose CRUD endpoints so administrators can update runtime-tunable settings—branding, feature flags, UI layouts, etc.—in the web UI without restarting services.

### **Motivation:**
- Today all defaults live in version-controlled *.conf files and require a service restart for any change.
- Enables safe, on-the-fly adjustments for branding, feature flags, and UI configuration via the Admin UI.
- Persisted overrides should live in site_settings and fall back to file defaults on first boot.

### **Proposal Document:**
[Configuration CRUD Design](https://docs.google.com/document/d/105EYiutoGCuuR-FPXUSzMgdTN7IEZEBNOM2IHBzKTxU/edit?tab=t.0)

### **Action Plan:**
- Migrate logos into DB & expose endpoints (#3511)
- Implement dynamic Sidebar Tabs flow
- Extend to additional configs