fix: handle NULL reboot_reason values from database

The reboot_reason field was defined as string instead of *string, causing
database scan failures when the column contains NULL values. This broke
agent list loading on existing installations after migration.

- Changed reboot_reason to *string in both Agent and AgentWithLastScan structs
- Added DEFAULT empty string to migration for new installations
- Added README section for full server reinstall procedure

aggregator-server/internal/database/migrations/013_add_reboot_tracking.up.sql

@@ -2,7 +2,7 @@
 ALTER TABLE agents
 ADD COLUMN reboot_required BOOLEAN DEFAULT FALSE,
 ADD COLUMN last_reboot_at TIMESTAMP,
-ADD COLUMN reboot_reason TEXT;
+ADD COLUMN reboot_reason TEXT DEFAULT '';
 
 -- Add index for efficient querying of agents needing reboot
 CREATE INDEX idx_agents_reboot_required ON agents(reboot_required) WHERE reboot_required = TRUE;