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/handlers/handlers_test.go

@@ -34,7 +34,6 @@ func TestHandleIndex(t *testing.T) {
 			logger.New,
 			func() *config.Config {
 				return &config.Config{
-					DBURL:   "file:" + t.TempDir() + "/test.db?cache=shared&mode=rwc",
 					DataDir: t.TempDir(),
 				}
 			},
@@ -66,7 +65,6 @@ func TestRenderTemplate(t *testing.T) {
 			logger.New,
 			func() *config.Config {
 				return &config.Config{
-					DBURL:   "file:" + t.TempDir() + "/test.db?cache=shared&mode=rwc",
 					DataDir: t.TempDir(),
 				}
 			},