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_test.go

@@ -24,7 +24,7 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "default is dev",
 			envValue:    "",
-			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
+			envVars:     map[string]string{},
 			expectError: false,
 			isDev:       true,
 			isProd:      false,
@@ -32,17 +32,15 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "explicit dev",
 			envValue:    "dev",
-			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
+			envVars:     map[string]string{},
 			expectError: false,
 			isDev:       true,
 			isProd:      false,
 		},
 		{
-			name:     "explicit prod",
-			envValue: "prod",
-			envVars: map[string]string{
-				"DBURL": "postgres://prod:prod@localhost:5432/prod?sslmode=require",
-			},
+			name:        "explicit prod",
+			envValue:    "prod",
+			envVars:     map[string]string{},
 			expectError: false,
 			isDev:       false,
 			isProd:      true,
@@ -50,7 +48,7 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "invalid environment",
 			envValue:    "staging",
-			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
+			envVars:     map[string]string{},
 			expectError: true,
 		},
 	}