DC-042: implement real Tailscale manager — replace null stub

The previous getTailscaleStatus() in src/app.js was a hard-coded
`return null` stub with a TODO saying it would be populated by context.
The context had a tailscale.* namespace declared with null function
stubs (routes/context.js:71), but nothing ever set them to real
functions. routes/tailscale.js has been calling ctx.tailscale.getStatus()
/ getLocalIP() / isTailscaleIP() and getting undefined back, silently
returning empty device lists. The tailscaleAuthMiddleware's allowedTailnet
check (DC-121, device-not-in-tailnet 403) was dead code for the same reason.

This commit replaces the stub with a real implementation:

- New src/managers/tailscale-manager.js shells out to the host's
  `tailscale status --json` (cached 5 minutes), parses the result, and
  exposes getStatus / getLocalIP / getSummary / getDevices / isTailscaleIP /
  invalidateCache / getAccessToken (stub) / startSyncTimer / stopSyncTimer
  / syncAPI (stub). All failure modes (CLI missing, tailscaled down,
  malformed JSON, EACCES) are handled gracefully — return null with no
  cache poisoning.
- src/context/index.js now wires the manager into ctx.tailscale.* so
  routes/tailscale.js and middleware.js's allowedTailnet gate get the
  real functions.
- src/app.js:189 getTailscaleStatus() now delegates to the manager
  instead of returning null.
- The duplicate isTailscaleIP() in src/app.js:179 (no malformed-input
  guards) is removed in favor of the canonical version in
  src/utilities/network-detector.js (DC-031) which the manager also uses.
- start.sh now bind-mounts /usr/bin/tailscale (statically linked Go binary
  — works under Alpine libc) and /var/run/tailscale/ into the container,
  read-only. Lets the container invoke the CLI without needing its own
  tailscale install.
- 41 new unit tests in __tests__/tailscale-manager.test.js cover: CLI
  success/missing/daemon-down/malformed-JSON paths, 5-min cache hit/miss,
  1-hour installed-cache hit/miss, getLocalIP IPv4/IPv6/missing-choices,
  getSummary shape, getDevices shape with full + minimal peer fields,
  startSyncTimer/stopSyncTimer interval + idempotency, TAILSCALE_BIN env
  override.

Total: 1138 tests pass (was 1097, +41 new), 0 new ESLint warnings.

What this unlocks:
- /api/v1/tailscale/status → real installed/connected/hostname/ip/
  peerCount/onlinePeerCount summary instead of empty
- /api/v1/tailscale/devices → real device list (was returning [])
- /api/v1/tailscale/check-connection → works (uses real isTailscaleIP)
- tailscaleAuthMiddleware allowedTailnet check (DC-121) is no longer
  dead code — a request from a Tailscale IP not in the allowed tailnet
  now actually gets 403 instead of being silently allowed.

start.sh

@@ -50,6 +50,13 @@ if docker ps -a --format "{{.Names}}" | grep -q "^${CONTAINER_NAME}$"; then
   docker rm -f ${CONTAINER_NAME}
 fi
 
+# Tailscale CLI + control socket — lets the container invoke
+# `tailscale status --json` to populate /api/v1/tailscale/status etc.
+# The binary is statically linked (Go), so the bind-mount works under
+# the container's Alpine libc without any library forwarding.
+# Both mounts are read-only: `tailscale status --json` is a read query
+# that the local tailscaled handles; we never need to mutate state
+# from inside the container.
 echo "[start.sh] Creating container with full config..."
 docker run -d --restart unless-stopped --name ${CONTAINER_NAME} \
   --add-host=get.dashcaddy.net:194.233.88.206 \
@@ -65,6 +72,8 @@ docker run -d --restart unless-stopped --name ${CONTAINER_NAME} \
   -v ${ASSETS_DIR}:/app/assets \
   -v ${UPDATES_DIR}:/app/updates \
   -v /opt/sami-files/logs:/opt/sami-files/logs:ro \
+  -v /usr/bin/tailscale:/usr/bin/tailscale:ro \
+  -v /var/run/tailscale:/var/run/tailscale:ro \
   -e NODE_ENV=production \
   -e SERVICES_FILE=/app/data/services.json \
   -e CONFIG_FILE=/app/data/config.json \