Add an admin-only server log viewer to the web UI

The root logger previously had no handler at all, so every module's
logger.info() call (user creation, claims, password resets, ...) was
silently dropped, not just unviewable. Adds a RotatingFileHandler
writing into the existing /data volume so log content also survives
container restarts/redeploys, plus /admin/logs (tail + line-count
picker + full-file download) alongside the existing Users & Frames
admin page.

server/README.md

@@ -91,11 +91,19 @@ algorithm itself -- it just streams the response straight to the panel.
   again until a recharge is detected and it crosses again. No SMTP
   configured, or no email on the relevant account, and both features
   silently no-op rather than erroring.
+- **Server logs.** `/admin/logs` shows the tail of the process's own
+  log file (`LOG_PATH` env var, default `/data/server.log` -- the same
+  `/data` volume as the database and legacy config, so it survives
+  container restarts/redeploys; `LOG_LEVEL` env var, default `INFO`).
+  Rotates at ~2MB x 3 backups; the page only reads the current file,
+  "Download full log" streams it raw. There's no log shipping/
+  aggregation beyond this -- it's a single-container deployment, so
+  the file *is* the log.
 
 ## Endpoints
 
 Pages: `/` (routing hub), `/setup`, `/login`, `/claim`, `/settings`,
-`/admin`, `/frames/{id}` (Photos), `/frames/{id}/config`,
+`/admin`, `/admin/logs`, `/frames/{id}` (Photos), `/frames/{id}/config`,
 `/frames/{id}/stats`, `/m/{manage_token}`.
 
 ### Device protocol (`/frame/*` -- paths frozen; auth = `?id=` + `?token=`)