Add SQLite database support design spec

docs/specs/2026-07-15-sqlite-database-design.md

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+# SQLite Database Support
+
+## Overview
+
+Add SQLite database support to Sis using the `sqlite-simple` library. On
+startup, the server opens a SQLite database (creating the file if it doesn't
+exist), enables WAL mode and foreign keys, and makes the connection available.
+No tables are created yet — that comes in a later task.
+
+## Architecture
+
+### New dependency
+
+- `sqlite-simple` added to `package.yaml`.
+
+### New module: `Sis.Database`
+
+Exposes one function:
+
+```haskell
+openDatabase :: FilePath -> IO Connection
+```
+
+Behavior:
+
+- Opens (or creates) the SQLite database file at the given path.
+- Enables WAL journal mode (`PRAGMA journal_mode=WAL`).
+- Enables foreign keys (`PRAGMA foreign_keys=ON`).
+- Returns the `Connection`.
+
+No tables are created in this task. The file will contain only the empty SQLite
+schema.
+
+### Server wiring
+
+- New CLI option `--db-path` in `app/Main.hs` with default value `data/sis.db`.
+- `main` opens the database connection via `Sis.Database.openDatabase` before
+  starting the Warp server.
+- The connection is not yet passed into the Orb app — that wiring happens when
+  the first table and routes are added in subsequent tasks.
+
+## Data flow
+
+```
+startup → parseOptions → openDatabase (creates file, sets pragmas)
+                       → start Warp server (connection held, unused for now)
+```
+
+## Error handling
+
+- If `openDatabase` fails (e.g., unwritable path, disk full), the exception
+  propagates and the server fails to start. This is correct — the server cannot
+  function without its database.
+
+## Testing
+
+- The existing `spec` test suite does not require changes since there are no
+  new routes or business logic.
+- Integration-level tests for database operations will be added when tables
+  and queries are introduced in subsequent tasks.