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/config/config.go

@@ -31,7 +31,6 @@ type ConfigParams struct {
 }
 
 type Config struct {
-	DBURL           string
 	DataDir         string
 	Debug           bool
 	MaintenanceMode bool
@@ -99,7 +98,6 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 
 	// Load configuration values from environment variables
 	s := &Config{
-		DBURL:           envString("DBURL"),
 		DataDir:         envString("DATA_DIR"),
 		Debug:           envBool("DEBUG", false),
 		MaintenanceMode: envBool("MAINTENANCE_MODE", false),
@@ -113,20 +111,16 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		params:          &params,
 	}
 
-	// Set default DataDir based on environment
+	// Set default DataDir based on environment. All SQLite databases
+	// (main application DB and per-webhook event DBs) live here.
 	if s.DataDir == "" {
 		if s.IsProd() {
-			s.DataDir = "/data/events"
+			s.DataDir = "/data"
 		} else {
 			s.DataDir = "./data"
 		}
 	}
 
-	// Validate database URL
-	if s.DBURL == "" {
-		return nil, fmt.Errorf("database URL (DBURL) is required")
-	}
-
 	if s.Debug {
 		params.Logger.EnableDebugLogging()
 	}