feat(documents): document_folders schema + folder_id on documents

Adds a per-port folder tree (self-FK on parent_id, unlimited depth)
plus a nullable folder_id on documents (null = root). Sibling-name
uniqueness enforced via a unique index on (port_id, COALESCE(parent_id,
'__root__'), LOWER(name)) so two folders can't share a name inside
the same parent. ON DELETE SET NULL on documents.folder_id and ON
DELETE NO ACTION on the parent self-FK so a botched delete never
silently destroys data — the service layer implements soft-rescue
(bubble children up to parent) instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/db/migrations/0050_document_folders.sql

@@ -0,0 +1,33 @@
+-- Document folders: per-port, unlimited-depth tree. parent_id references
+-- another document_folders row; null = root. Sibling-name uniqueness is
+-- enforced via a partial-uniqueness on (port_id, COALESCE(parent_id,
+-- '__root__'), LOWER(name)) so two folders can't share a name inside
+-- the same parent. The CRM checks parent_id chain for cycles in the
+-- service layer; no DB-side cycle guard.
+CREATE TABLE IF NOT EXISTS "document_folders" (
+  "id" text PRIMARY KEY NOT NULL,
+  "port_id" text NOT NULL REFERENCES "ports" ("id"),
+  "parent_id" text,
+  "name" text NOT NULL,
+  "created_by" text NOT NULL,
+  "created_at" timestamp with time zone NOT NULL DEFAULT now(),
+  "updated_at" timestamp with time zone NOT NULL DEFAULT now()
+);
+
+ALTER TABLE "document_folders"
+  ADD CONSTRAINT "document_folders_parent_fk"
+  FOREIGN KEY ("parent_id") REFERENCES "document_folders" ("id")
+  ON DELETE NO ACTION;
+
+CREATE INDEX IF NOT EXISTS "idx_document_folders_port"
+  ON "document_folders" ("port_id");
+CREATE INDEX IF NOT EXISTS "idx_document_folders_parent"
+  ON "document_folders" ("parent_id");
+CREATE UNIQUE INDEX IF NOT EXISTS "uniq_document_folders_sibling_name"
+  ON "document_folders" ("port_id", COALESCE("parent_id", '__root__'), LOWER("name"));
+
+ALTER TABLE "documents"
+  ADD COLUMN IF NOT EXISTS "folder_id" text REFERENCES "document_folders" ("id") ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS "idx_docs_folder"
+  ON "documents" ("folder_id");