doc: add design docs for open epics

- Omni/Task/DESIGN.md: Task manager improvements -
Biz/PodcastItLater/DESIGN.md: Architecture and features
- Biz/PodcastItLater/TESTING.md: Test strategy -
Network/Wai/Middleware/Braid/DESIGN.md: Keep-alive design

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+# Braid Middleware Design
+
+**Goal**: Implement HTTP Keep-Alive mechanism for Braid updates to support real-time streams.
+
+## Current State
+`Network/Wai/Middleware/Braid.hs` implements the Braid protocol headers but lacks a robust mechanism to keep the connection open and push updates.
+
+## Design Requirements
+
+1.  **Connection Management**:
+    - Identify Braid subscriptions via headers.
+    - Keep the response body open (streaming response).
+    - Handle client disconnects gracefully.
+
+2.  **Update Channel**:
+    - Use a `TChan` or `BroadcastChan` to signal updates to the connection handler.
+    - When a resource changes, push a new Braid frame to the open stream.
+
+3.  **Frame Format**:
+    - Adhere to Braid spec for patch frames.
+    - `Content-Type: application/json` (or relevant type).
+    - `Merge-Type: braid`.
+
+## Implementation Plan
+1.  Modify middleware to hijack response for Braid requests.
+2.  Spawn a lightweight thread to listen on an update channel.
+3.  Stream chunks to the client.