docs: add per-module READMEs with architecture and DB schemas

Each module now has a README.md documenting commands, architecture,
and database schema (KV keys, JSON structure, field descriptions).
Trading README enhanced with full schemas for user portfolio and
price cache objects.

src/modules/misc/README.md

@@ -0,0 +1,35 @@
+# Misc Module
+
+Health check and DB demonstration. Exercises all three visibility levels.
+
+## Commands
+
+| Command | Visibility | Description |
+|---------|-----------|-------------|
+| `/ping` | public | Health check — replies "pong" and records timestamp |
+| `/mstats` | protected | Shows last `/ping` timestamp |
+| `/fortytwo` | private | Easter egg — replies "The answer." |
+
+## Architecture
+
+- `/ping` writes a timestamp to KV on every call (best-effort — KV failure doesn't block the reply).
+- `/mstats` reads the last ping timestamp and formats it as ISO string.
+- `/fortytwo` is a hidden command demonstrating private visibility.
+
+## Database
+
+KV namespace prefix: `misc:`
+
+| Key | Type | Description | Example |
+|-----|------|-------------|---------|
+| `last_ping` | JSON | Timestamp of the most recent `/ping` call | `{ "at": 1713100000000 }` |
+
+### Schema: `last_ping`
+
+```json
+{
+  "at": 1713100000000
+}
+```
+
+- `at` — Unix epoch milliseconds (`Date.now()`)