Make AI categorization prompts user-configurable

Add SystemPrompt and UserPromptTemplate fields to AI categorization
settings so users can customize the prompts sent to Ollama without
code changes. Null/empty values fall back to built-in defaults.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Data/Configurations/AiCategorizationSettingsConfiguration.cs

@@ -12,6 +12,8 @@ public class AiCategorizationSettingsConfiguration : IEntityTypeConfiguration<Ai
         builder.HasKey(s => s.Id);
         builder.Property(s => s.OllamaUrl).HasMaxLength(500);
         builder.Property(s => s.ModelName).HasMaxLength(100);
+        builder.Property(s => s.SystemPrompt).HasMaxLength(4000);
+        builder.Property(s => s.UserPromptTemplate).HasMaxLength(4000);
 
         builder.HasOne(s => s.User)
             .WithOne(u => u.AiCategorizationSettings)

src/Purrse.Data/Migrations/20260209002801_AddAiPromptTemplates.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Purrse.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddAiPromptTemplates : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "SystemPrompt",
+                table: "ai_categorization_settings",
+                type: "character varying(4000)",
+                maxLength: 4000,
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "UserPromptTemplate",
+                table: "ai_categorization_settings",
+                type: "character varying(4000)",
+                maxLength: 4000,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "SystemPrompt",
+                table: "ai_categorization_settings");
+
+            migrationBuilder.DropColumn(
+                name: "UserPromptTemplate",
+                table: "ai_categorization_settings");
+        }
+    }
+}

src/Purrse.Data/Migrations/PurrseDbContextModelSnapshot.cs

@@ -107,6 +107,10 @@ namespace Purrse.Data.Migrations
                         .HasMaxLength(500)
                         .HasColumnType("character varying(500)");
 
+                    b.Property<string>("SystemPrompt")
+                        .HasMaxLength(4000)
+                        .HasColumnType("character varying(4000)");
+
                     b.Property<int>("TimeoutSeconds")
                         .HasColumnType("integer");
 
@@ -119,6 +123,10 @@ namespace Purrse.Data.Migrations
                     b.Property<Guid>("UserId")
                         .HasColumnType("uuid");
 
+                    b.Property<string>("UserPromptTemplate")
+                        .HasMaxLength(4000)
+                        .HasColumnType("character varying(4000)");
+
                     b.HasKey("Id");
 
                     b.HasIndex("UserId")