Add GitHub Copilot prompt files

- Step-by-step prompts for solution skeleton, domain model,
  DbContext mappings, application services, UI shell/pages,
  PDF export, and tests/validation

.github/prompts/03-dbcontext-mappings.prompt.md

@@ -0,0 +1,44 @@
+---
+description: "Implement FinanceDbContext with fluent entity configurations, indexes, relationships, and SQLite connection setup"
+agent: "agent"
+---
+
+# Step 3: DbContext & Mappings
+
+Create `src/Duempelkas.Infrastructure/Persistence/FinanceDbContext.cs`.
+
+## DbSets
+- `DbSet<Account> Accounts`
+- `DbSet<AccountYear> AccountYears`
+- `DbSet<Entry> Entries`
+- `DbSet<TransferLink> TransferLinks`
+
+## Connection
+- Accept SQLite connection string via constructor options.
+- Default path: `Data Source=duempelkas.db` in the app's base directory.
+
+## Entity Configurations (use `OnModelCreating` with `IEntityTypeConfiguration<T>` or inline)
+
+### Account
+- `Name` required, max length 200.
+- Cascade delete → AccountYears.
+
+### AccountYear
+- Unique index on `(AccountId, Year)`.
+- `OpeningBalance` column type `decimal(18,2)`.
+- Cascade delete → Entries.
+
+### Entry
+- Index on `(AccountYearId, Date)`.
+- `Amount` column type `decimal(18,2)`.
+- `Type` stored as int.
+- Optional FK to TransferLink (`TransferLinkId`), restrict delete.
+
+### TransferLink
+- Unique index on `SourceEntryId`.
+- Unique index on `TargetEntryId`.
+- Relationships to SourceEntry and TargetEntry with `DeleteBehavior.Restrict`.
+
+## Notes
+- Use separate `IEntityTypeConfiguration<T>` classes in `Persistence/Configurations/`.
+- Do not auto-generate migrations; the initial migration will be created manually later.