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/database_test.go

@@ -2,7 +2,6 @@ package database
 
 import (
 	"context"
-	"os"
 	"testing"
 
 	"go.uber.org/fx/fxtest"
@@ -12,10 +11,6 @@ import (
 )
 
 func TestDatabaseConnection(t *testing.T) {
-	// Set DBURL env var for config loading
-	os.Setenv("DBURL", "file::memory:?cache=shared")
-	defer os.Unsetenv("DBURL")
-
 	// Set up test dependencies
 	lc := fxtest.NewLifecycle(t)
 
@@ -35,18 +30,12 @@ func TestDatabaseConnection(t *testing.T) {
 		t.Fatalf("Failed to create logger: %v", err)
 	}
 
-	// Create config
-	c, err := config.New(lc, config.ConfigParams{
-		Globals: g,
-		Logger:  l,
-	})
-	if err != nil {
-		t.Fatalf("Failed to create config: %v", err)
+	// Create config with DataDir pointing to a temp directory
+	c := &config.Config{
+		DataDir:     t.TempDir(),
+		Environment: "dev",
 	}
 
-	// Override DBURL to use a temp file-based SQLite (in-memory doesn't persist across connections)
-	c.DBURL = "file:" + t.TempDir() + "/test.db?cache=shared&mode=rwc"
-
 	// Create database
 	db, err := New(lc, DatabaseParams{
 		Config: c,