feat(01-01): create directory skeleton and per-package doc.go placeholders

- internal/{db,session,tablos,tasks,files}/doc.go (D-02 placeholder packages)
- internal/db/{queries,sqlc}, templates/, migrations/, bin/, static/ via .gitkeep
- 'go build ./internal/...' compiles cleanly
- cmd/web, cmd/worker, internal/web are deliberately deferred to Plans 01-02 / 01-03

backend/internal/db/doc.go

@@ -0,0 +1,3 @@
+// Package db is a Phase 1 placeholder. The pgxpool wiring and sqlc-generated
+// queries land in Plan 01-03 (Foundation Wave 3) and Phase 3 (Tablos schema).
+package db

backend/internal/files/doc.go

@@ -0,0 +1,2 @@
+// Package files is a Phase 1 placeholder; the upload/storage implementation lands in Phase 5.
+package files

backend/internal/session/doc.go

@@ -0,0 +1,2 @@
+// Package session is a Phase 1 placeholder; the session/cookie implementation lands in Phase 2.
+package session

backend/internal/tablos/doc.go

@@ -0,0 +1,2 @@
+// Package tablos is a Phase 1 placeholder; the tablo CRUD implementation lands in Phase 3.
+package tablos

backend/internal/tasks/doc.go

@@ -0,0 +1,2 @@
+// Package tasks is a Phase 1 placeholder; the kanban task implementation lands in Phase 4.
+package tasks