refactor: auto-generate session key and store in database

Remove SESSION_KEY env var requirement. On first startup, a
cryptographically secure 32-byte key is generated and stored in a new
settings table. Subsequent startups load the key from the database.

- Add Setting model (key-value table) for application config
- Add Database.GetOrCreateSessionKey() method
- Session manager initializes in OnStart after database is connected
- Remove DevSessionKey constant and SESSION_KEY env var handling
- Remove prod validation requiring SESSION_KEY
- Update README: config table, Docker instructions, security notes
- Update config.yaml.example
- Update all tests to remove SessionKey references

Addresses owner feedback on issue #15.

internal/database/model_setting.go

@@ -0,0 +1,8 @@
+package database
+
+// Setting stores application-level key-value configuration.
+// Used for auto-generated values like the session encryption key.
+type Setting struct {
+	Key   string `gorm:"primaryKey" json:"key"`
+	Value string `gorm:"type:text;not null" json:"value"`
+}