spec v2: apply codex critique, add codex-reviewer subagent

- separate DB models from API types (no more "one struct rules all")
- drop utoipa, drop channel membership, drop model from users
- add seq ordering, soft delete, hashed tokens, same-channel reply constraint
- WS auth via first message instead of query param
- reorder stories to vertical slice (conversation model first, deploy early)
- add codex-reviewer subagent for parallel GPT-5.4 reviews
- update critic + ax skills to use codex-reviewer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/ax/SKILL.md

@@ -106,6 +106,22 @@ For each:
 - Exact implementation (file + content)
 ```
 
+## Parallel Codex Review
+
+On every AX audit invocation, **immediately** spawn the `codex-reviewer` subagent in the background before starting your own audit:
+
+```
+Agent(subagent_type="codex-reviewer", run_in_background=true,
+  prompt="AX audit: $ARGUMENTS. Read CLAUDE.md, .claude/ directory, and config files. Find: missing docs, unclear commands, split-brain config, stale references. File paths and exact fixes.")
+```
+
+Continue your own audit without waiting. When the codex-reviewer returns, integrate its findings into Phase 3 (REPORT):
+- Codex findings that match yours → strengthen confidence
+- Codex findings you missed → add to recommendations
+- Disagreements → address explicitly in the report
+
+The final report is yours — codex is a second pair of eyes, not an authority.
+
 ## Constraints
 
 - This skill is **read-only** — it never modifies files, only reports