feat: implement core webhook engine, delivery system, and management UI (Phase 2)

- Webhook reception handler: look up entrypoint by UUID, verify active,
  capture full HTTP request (method, headers, body, content-type), create
  Event record, queue Delivery records for each active Target, return 200 OK.
  Handles edge cases: unknown UUID → 404, inactive → 410, oversized → 413.

- Delivery engine (internal/delivery): fx-managed background goroutine that
  polls for pending/retrying deliveries and dispatches to target type handlers.
  Graceful shutdown via context cancellation.

- Target type implementations:
  - HTTP: fire-and-forget POST with original headers forwarding
  - Retry: exponential backoff (1s, 2s, 4s...) up to max_retries
  - Database: immediate success (event already stored)
  - Log: slog output with event details

- Webhook management pages with Tailwind CSS + Alpine.js:
  - List (/sources): webhooks with entrypoint/target/event counts
  - Create (/sources/new): form with auto-created default entrypoint
  - Detail (/source/{id}): config, entrypoints, targets, recent events
  - Edit (/source/{id}/edit): name, description, retention_days
  - Delete (/source/{id}/delete): soft-delete with child records
  - Add Entrypoint (/source/{id}/entrypoints): inline form
  - Add Target (/source/{id}/targets): type-aware form
  - Event Log (/source/{id}/logs): paginated with delivery status

- Updated README: marked completed items, updated naming conventions
  table, added delivery engine to package layout and DI docs, updated
  column names to reflect entity rename.

- Rebuilt Tailwind CSS for new template classes.

Part of: #15

internal/handlers/handlers.go

@@ -53,9 +53,14 @@ func New(lc fx.Lifecycle, params HandlersParams) (*Handlers, error) {
 
 	// Parse all page templates once at startup
 	s.templates = map[string]*template.Template{
-		"index.html":   parsePageTemplate("index.html"),
-		"login.html":   parsePageTemplate("login.html"),
-		"profile.html": parsePageTemplate("profile.html"),
+		"index.html":         parsePageTemplate("index.html"),
+		"login.html":         parsePageTemplate("login.html"),
+		"profile.html":       parsePageTemplate("profile.html"),
+		"sources_list.html":  parsePageTemplate("sources_list.html"),
+		"sources_new.html":   parsePageTemplate("sources_new.html"),
+		"source_detail.html": parsePageTemplate("source_detail.html"),
+		"source_edit.html":   parsePageTemplate("source_edit.html"),
+		"source_logs.html":   parsePageTemplate("source_logs.html"),
 	}
 
 	lc.Append(fx.Hook{