Centralize Postgres connection string policy across all modules

Extract connection string building into PostgresConnectionStringPolicy so all
services use consistent pooling, application_name, and timeout settings.
Adopt the new policy in 20+ module DataSource/ServiceCollectionExtensions classes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/__Libraries/StellaOps.Infrastructure.Postgres/Options/PostgresOptions.cs

@@ -20,6 +20,11 @@ public sealed class PostgresOptions
     /// </summary>
     public int MaxPoolSize { get; set; } = 100;
 
+    /// <summary>
+    /// Stable PostgreSQL application name used for runtime attribution.
+    /// </summary>
+    public string? ApplicationName { get; set; }
+
     /// <summary>
     /// Minimum number of connections in the pool. Default is 1.
     /// </summary>