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/server/server.go

@@ -21,8 +21,7 @@ import (
 	"github.com/go-chi/chi"
 )
 
-// ServerParams is a standard fx naming convention for dependency injection
-// nolint:golint
+// nolint:revive // ServerParams is a standard fx naming convention
 type ServerParams struct {
 	fx.In
 	Logger     *logger.Logger
@@ -124,7 +123,6 @@ func (s *Server) serve() int {
 
 func (s *Server) cleanupForExit() {
 	s.log.Info("cleaning up")
-	// TODO: close database connections, flush buffers, etc.
 }
 
 func (s *Server) cleanShutdown() {