feat(server): persist node semantic context and retain CAD agent context

Add a semantic_context column to canvas_nodes (schema, sqlc queries,
generated models, repository upserts) and thread SemanticContext through the
domain node, API type, and mappers so CAD-derived context survives canvas
saves.

Treat agent text parts with textSource "cad-context" like settings: keep
them for planning, memory, and image-generation prompts but exclude them
from the persisted user-visible message copy. Add coverage for the CAD
context visibility and mapper behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/infrastructure/postgres/schema.sql

@@ -194,7 +194,8 @@ CREATE TABLE IF NOT EXISTS canvas_nodes (
     stroke_style TEXT NOT NULL DEFAULT '',
     flip_x BOOLEAN NOT NULL DEFAULT FALSE,
     flip_y BOOLEAN NOT NULL DEFAULT FALSE,
-    rotation DOUBLE PRECISION NOT NULL DEFAULT 0
+    rotation DOUBLE PRECISION NOT NULL DEFAULT 0,
+    semantic_context TEXT NOT NULL DEFAULT ''
 );
 
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS user_id UUID;
@@ -225,6 +226,7 @@ ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS stroke_style TEXT NOT NULL DEF
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS flip_x BOOLEAN NOT NULL DEFAULT FALSE;
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS flip_y BOOLEAN NOT NULL DEFAULT FALSE;
 ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS rotation DOUBLE PRECISION NOT NULL DEFAULT 0;
+ALTER TABLE canvas_nodes ADD COLUMN IF NOT EXISTS semantic_context TEXT NOT NULL DEFAULT '';
 
 CREATE INDEX IF NOT EXISTS canvas_nodes_project_id_idx ON canvas_nodes(project_id);
 CREATE INDEX IF NOT EXISTS canvas_nodes_user_project_idx ON canvas_nodes(user_id, project_id);