feat: controller-side HTTP/TCP health probes

Add network-level health probing from the controller to deployed apps.
The controller probes containers over the shared Docker network and
overrides stack state to "unhealthy" if the service isn't responding.

Three probe types: http (any response = alive), api (validates status
code and body content), tcp (port reachability). Configured per-app
via healthcheck: section in .felhom.yml. Runs every minute, per-app
interval defaults to 5 minutes.

This replaces Docker-level healthchecks for distroless images (e.g.
Vikunja) that lack shell utilities, and complements existing Docker
healthchecks for other apps.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,16 @@
 ## Changelog
 
+### v0.29.3 — Controller-side Health Probes (2026-02-25)
+
+#### Added
+- **HTTP/TCP health probes** (`internal/stacks/healthprobe.go`) — The controller now probes deployed apps directly over the Docker network to verify services are actually responding, not just that containers are running. Runs every minute, configurable per-app interval (default 5 min).
+- **Three probe types**: `http` (any response = alive), `api` (validates status code and response body), `tcp` (port reachability). Multiple checks per app supported.
+- **`.felhom.yml` healthcheck config** (`internal/stacks/metadata.go`) — New `healthcheck:` section with `interval`, `checks[]` (type, port, path, method, expect). Parsed from app catalog metadata.
+- **State override** (`internal/stacks/manager.go`) — If a running container's health probe fails, the stack state is overridden to "unhealthy". Clears automatically when probe passes again.
+
+#### Fixed
+- **Vikunja healthcheck** — Removed Docker-level healthcheck (distroless image has no wget/curl). Controller-side API probe to `:3456/api/v1/info` replaces it.
+
 ### v0.29.2 — Dynamic Logo & Favicon (2026-02-25)
 
 #### Changed