feat(alarms): HiLo trigger type with per-band level, hysteresis, messages, overrides

Adds a new HiLo alarm trigger type with four configurable setpoints
(LoLo / Lo / Hi / HiHi). Each setpoint carries an optional priority,
deadband (for hysteresis), and operator message. The site runtime emits
AlarmStateChanged with an AlarmLevel field so consumers can differentiate
warning vs critical bands.

Plumbing:
  - new AlarmLevel enum + AlarmStateChanged.Level/Message init properties
  - AlarmTriggerEditor (Blazor) gets a HiLo render with severity tinting
  - AlarmTriggerConfigCodec extracted from the editor for testability
  - sitestream.proto carries level + message over gRPC
  - SemanticValidator enforces numeric attribute, setpoint ordering,
    non-negative deadband
  - on-trigger scripts get an Alarm global (Name/Level/Priority/Message)
    so notification routing can branch by severity
  - per-instance InstanceAlarmOverride entity + EF migration + flattening
    step + CLI commands; HiLo overrides merge setpoint-by-setpoint, binary
    types whole-replace
  - DebugView shows a Level badge + per-band message tooltip
  - App.razor auto-reloads on permanent Blazor circuit failure
  - docker/regen-proto.sh automates the proto regen workflow (the linux/arm64
    protoc segfault means generated files are checked in for now)

src/ScadaLink.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -30,6 +30,7 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Instances
     public DbSet<Instance> Instances => Set<Instance>();
     public DbSet<InstanceAttributeOverride> InstanceAttributeOverrides => Set<InstanceAttributeOverride>();
+    public DbSet<InstanceAlarmOverride> InstanceAlarmOverrides => Set<InstanceAlarmOverride>();
     public DbSet<InstanceConnectionBinding> InstanceConnectionBindings => Set<InstanceConnectionBinding>();
     public DbSet<Area> Areas => Set<Area>();