test: add tests for delivery, middleware, and session packages (#32)

## Summary

Add comprehensive test coverage for three previously-untested packages, addressing [issue #28](#28).

## Coverage Improvements

| Package | Before | After |
|---------|--------|-------|
| `internal/delivery` | 37.1% | 74.5% |
| `internal/middleware` | 0.0% | 70.2% |
| `internal/session` | 0.0% | 51.5% |

## What's Tested

### delivery (37% → 75%)
- `processNewTask` with inline and large (DB-fetched) bodies
- `processRetryTask` success, skip non-retrying, large body fetch
- Worker lifecycle start/stop, retry channel processing
- `processDelivery` unknown target type handling
- `recoverPendingDeliveries`, `recoverWebhookDeliveries`, `recoverInFlight`
- HTTP delivery with custom headers, timeout, invalid config
- `Notify` batching

### middleware (0% → 70%)
- Logging middleware status code capture and pass-through
- `LoggingResponseWriter` delegation
- CORS dev mode (allow-all) and prod mode (no-op)
- `RequireAuth` redirect for unauthenticated, pass-through for authenticated
- `MetricsAuth` basic auth validation
- `ipFromHostPort` helper

### session (0% → 52%)
- `Get`/`Save` round-trip with real cookie store
- `SetUser`, `GetUserID`, `GetUsername`, `IsAuthenticated`
- `ClearUser` removes all keys
- `Destroy` invalidates session (MaxAge -1)
- Session persistence across requests
- Edge cases: overwrite user, wrong type, constants

## Test Helpers Added
- `database.NewTestDatabase` / `NewTestWebhookDBManager` — cross-package test helpers for delivery integration tests
- `session.NewForTest` — creates session manager without fx lifecycle for middleware tests

## Notes
- No production code modified
- All tests use `httptest`, SQLite in-memory, and real cookie stores — no external network calls
- Full test suite completes in ~3.5s within the 30s timeout
- `docker build .` passes (lint + test + build)

closes #28

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #32
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/database/testing.go

@@ -0,0 +1,28 @@
+package database
+
+import (
+	"log/slog"
+	"os"
+
+	"gorm.io/gorm"
+)
+
+// NewTestDatabase creates a Database wrapper around a pre-opened *gorm.DB.
+// Intended for use in tests that need a *database.Database without the
+// full fx lifecycle. The caller is responsible for closing the underlying
+// sql.DB connection.
+func NewTestDatabase(db *gorm.DB) *Database {
+	return &Database{
+		db:  db,
+		log: slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelDebug})),
+	}
+}
+
+// NewTestWebhookDBManager creates a WebhookDBManager backed by the given
+// data directory. Intended for use in tests without the fx lifecycle.
+func NewTestWebhookDBManager(dataDir string) *WebhookDBManager {
+	return &WebhookDBManager{
+		dataDir: dataDir,
+		log:     slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelDebug})),
+	}
+}