Add read-only companion endpoints (ping/info/owner-scoped models) (#863)

First, smallest cut of a LAN companion bridge (split out of #855 per review):
a thin, additive, read-only layer so a LAN client can discover what a server
offers. No new LLM logic; auth is enforced by the existing AuthMiddleware.

- GET /api/companion/ping  -- cheap auth-validated health check
- GET /api/companion/info  -- server identity + capability flags
- GET /api/companion/models -- the CALLER's own model endpoints

/models scopes to the caller's real owner (the token's owner for bearer callers)
plus legacy null-owner shared rows, mirroring owner_filter, and never returns
api_key material. The owner rule lives in two pure helpers (token_owner,
owner_can_see) with direct tests proving a token for owner A cannot see owner B's
rows and that null-owner rows don't widen access.

companion/README.md

@@ -0,0 +1,20 @@
+# Companion bridge (read-only)
+
+A thin, additive layer so a LAN client can discover what an Odysseus server
+offers, without duplicating any LLM logic. Reachable with either a logged-in
+cookie session or a Bearer `ody_` API token (auth is enforced globally by
+`AuthMiddleware`).
+
+| Method | Path | Purpose |
+|---|---|---|
+| GET | `/api/companion/ping` | cheap, auth-validated health check |
+| GET | `/api/companion/info` | server identity + capability flags |
+| GET | `/api/companion/models` | the **caller's own** model endpoints |
+
+`/models` scopes to the caller's real owner (the token's owner for bearer
+callers) plus legacy null-owner shared rows, the same rule as `owner_filter`. It
+never returns API-key material. The owner rule lives in two pure, tested helpers
+(`token_owner`, `owner_can_see`) — see `tests/test_companion_readonly.py`.
+
+This module is intentionally read-only. Pairing/token-minting, token-owner
+session attribution, and any mutation endpoints are proposed in separate PRs.