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/server/routes.go

@@ -100,11 +100,13 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Route("/source/{sourceID}", func(r chi.Router) {
 		r.Use(s.mw.RequireAuth())
-		r.Get("/", s.h.HandleSourceDetail())          // View webhook details
-		r.Get("/edit", s.h.HandleSourceEdit())        // Show edit form
-		r.Post("/edit", s.h.HandleSourceEditSubmit()) // Handle edit submission
-		r.Post("/delete", s.h.HandleSourceDelete())   // Delete webhook
-		r.Get("/logs", s.h.HandleSourceLogs())        // View webhook logs
+		r.Get("/", s.h.HandleSourceDetail())                 // View webhook details
+		r.Get("/edit", s.h.HandleSourceEdit())               // Show edit form
+		r.Post("/edit", s.h.HandleSourceEditSubmit())        // Handle edit submission
+		r.Post("/delete", s.h.HandleSourceDelete())          // Delete webhook
+		r.Get("/logs", s.h.HandleSourceLogs())               // View webhook logs
+		r.Post("/entrypoints", s.h.HandleEntrypointCreate()) // Add entrypoint
+		r.Post("/targets", s.h.HandleTargetCreate())         // Add target
 	})
 
 	// Entrypoint endpoint - accepts incoming webhook POST requests