feat(messages): attach files and share appointments

- new message_attachments table + POST/GET /api/conversations/attachments
  (base64 upload, capped 10MB; authed clinic-scoped download)
- messages carry an attachments JSON column (file refs or appointment
  snapshots); realtime + REST send accept attachments and allow
  attachment-only messages; migration 0019
- composer "+" menu (Files / Appointments): file picker uploads and
  stages chips; appointment picker searches by patient and attaches a
  snapshot; both render in the thread (download chip / appointment card)
- raise express json limit to 15mb for base64 uploads

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/drizzle/0019_black_mole_man.sql

@@ -0,0 +1,16 @@
+CREATE TABLE "message_attachments" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"organization_id" text NOT NULL,
+	"uploader_id" text,
+	"file_name" text NOT NULL,
+	"mime_type" text NOT NULL,
+	"size" integer NOT NULL,
+	"data" text NOT NULL,
+	"created_at" timestamp DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+ALTER TABLE "messages" ALTER COLUMN "body" SET DEFAULT '';--> statement-breakpoint
+ALTER TABLE "messages" ADD COLUMN "attachments" jsonb;--> statement-breakpoint
+ALTER TABLE "message_attachments" ADD CONSTRAINT "message_attachments_organization_id_organization_id_fk" FOREIGN KEY ("organization_id") REFERENCES "public"."organization"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "message_attachments" ADD CONSTRAINT "message_attachments_uploader_id_user_id_fk" FOREIGN KEY ("uploader_id") REFERENCES "public"."user"("id") ON DELETE set null ON UPDATE no action;--> statement-breakpoint
+CREATE INDEX "message_attachments_org_idx" ON "message_attachments" USING btree ("organization_id");

backend/drizzle/meta/_journal.json

@@ -134,6 +134,13 @@
       "when": 1781537902724,
       "tag": "0018_clean_doctor_strange",
       "breakpoints": true
+    },
+    {
+      "idx": 19,
+      "version": "7",
+      "when": 1781538797034,
+      "tag": "0019_black_mole_man",
+      "breakpoints": true
     }
   ]
 }