chore: consolidate DBURL into DATA_DIR, codebase audit for 1.0.0

DBURL → DATA_DIR consolidation:
- Remove DBURL env var entirely; main DB now lives at {DATA_DIR}/webhooker.db
- database.go constructs DB path from config.DataDir, ensures dir exists
- Update DATA_DIR prod default from /data/events to /data
- Update all tests to use DataDir instead of DBURL
- Update Dockerfile: /data (not /data/events) for all SQLite databases
- Update README configuration table, Docker examples, architecture docs

Dead code removal:
- Remove unused IndexResponse struct (handlers/index.go)
- Remove unused TemplateData struct (handlers/handlers.go)

Stale comment cleanup:
- Remove TODO in server.go (DB cleanup handled by fx lifecycle)
- Fix nolint:golint → nolint:revive on ServerParams for consistency
- Clean up verbose middleware/routing comments in routes.go
- Fix TODO fan-out description (worker pool, not goroutine-per-target)

.gitignore fixes:
- Add data/ directory to gitignore
- Remove stale config.yaml entry (env-only config since rework)

internal/database/webhook_db_manager_test.go

@@ -18,10 +18,6 @@ import (
 func setupTestWebhookDBManager(t *testing.T) (*WebhookDBManager, *fxtest.Lifecycle) {
 	t.Helper()
 
-	// Set DBURL env var for config loading
-	os.Setenv("DBURL", "file::memory:?cache=shared")
-	t.Cleanup(func() { os.Unsetenv("DBURL") })
-
 	lc := fxtest.NewLifecycle(t)
 
 	globals.Appname = "webhooker-test"
@@ -37,7 +33,6 @@ func setupTestWebhookDBManager(t *testing.T) (*WebhookDBManager, *fxtest.Lifecyc
 	dataDir := filepath.Join(t.TempDir(), "events")
 
 	cfg := &config.Config{
-		DBURL:   "file::memory:?cache=shared",
 		DataDir: dataDir,
 	}