refactor(gui): clean up flagged issues before merge

1. Make migration 015 idempotent with IF NOT EXISTS

2. Remove hardcoded cadence range from routes.py and template:
   - Added ge=10 constraint to AdapterConfig.cadence_s field
   - Removed manual 60-3600 check from routes.py POST handler
   - Validate cadence using AdapterConfig field metadata
   - Removed min/max attributes from template input

3. Move discover_adapters to its own module:
   - Created src/central/adapter_discovery.py
   - Updated supervisor.py to import from adapter_discovery
   - Updated routes.py to import from adapter_discovery
   - GUI no longer transitively imports nats or stream_manager

4. Remove dead code branch in form_descriptors.py:
   - Removed unreachable RegionConfig check (already handled earlier)
   - Improved error message for unsupported nested types

5. Updated test_adapters.py:
   - Changed invalid cadence test from 30 to 5 (below ge=10)
   - Updated assertion to check for "10" in error message

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sql/migrations/015_add_adapters_last_error.sql

@@ -3,4 +3,4 @@
 -- Populated by supervisor when an adapter fails to start or apply config.
 
 ALTER TABLE config.adapters
-ADD COLUMN last_error TEXT;
+ADD COLUMN IF NOT EXISTS last_error TEXT;