implemented 09-system-metrics.md: added system metrics: introduced system monitoring with a metrics collector, REST API endpoint, Thymeleaf UI rendering, and lifecycle management

docs/plan/09-system-metrics.md

@@ -33,3 +33,30 @@ Create package `de.hoennig.gittally.metrics`:
 
 - `./gradlew ktlintFormat` then `./gradlew build` is green.
 - `/system` renders live values on Linux (manual smoke test; document in this file).
+
+## Implementation Notes (2026-07-07)
+
+Implemented as designed: `SystemMetricsCollector` in `de.hoennig.gittally.metrics` samples every 60s once `ServerMetricsLifecycle` (server profile only) calls `start()`, following the watcher's start/stop pattern.
+CPU comes from `/proc/stat` deltas, RAM from `/proc/meminfo`, disk from `java.nio.file.FileStore` (`df` semantics: used = total − unallocated, free = usable), and the repository size from a file walk.
+`GET /api/system` returns the snapshot plus aggregates, and `/system` renders the legacy system page in the step 08 layout, polling every 60s with the same timeout/error-badge rules.
+Since the metric rows are fixed, `gittally.js` only updates the cell texts in place — nothing is rebuilt.
+
+Deviations and decisions:
+
+- The JSON uses camelCase fields with nested `{current, min, max, avg}` aggregates instead of the flat snake_case legacy `system.json`; the value set matches legacy.
+- The aggregation state persists as `system-metrics-state.json` in the artifact root and restarts continue the series, as this step requires.
+  Legacy actually deleted `system_state.dat` on every start, so the footnote now reads "since the first server start" instead of "since script start".
+  `ArtifactStore` gained `rootDir()` so the state can live next to the stored builds.
+- CPU load needs a counter delta, so the first sample after process start reports no CPU metric yet (`n/a`); legacy aggregated a meaningless near-zero first delta instead.
+- The repository size is re-probed only every 10th sample (10 minutes) and reused in between — the throttle this step requires; legacy ran `du -sk` every cycle.
+  The file walk sums file sizes, not disk blocks like `du`, which is close enough for a trend metric.
+- An unavailable source (no `/proc` outside Linux, unreadable file store) yields explicit `null` metrics over HTTP 200 and `n/a` cells; the failure is logged once, not every 60s.
+- No new config keys: the 60s interval is fixed like legacy, so `GitTallyConfig`, the `init` templates, and `docs/configuration.md` are unchanged.
+- The legacy `generation` field was not ported; it only guarded the legacy JS against monitor restarts.
+- The CPU count comes from `Runtime.availableProcessors()` instead of `nproc`.
+
+Manual smoke test (2026-07-07): scratch repository with a bare origin, server on port 18986, observed through a real browser tab (via a TCP proxy, so the tab outlived backend restarts).
+The first sample rendered RAM/disk/repo values immediately with CPU `n/a` and the totals line (`8 cores`, RAM/disk GiB, updated time).
+After the next 60s poll the open tab updated in place without reload: the updated time ticked, CPU used appeared (1.58 cores, idle 6.42 = 8 total), and min/max diverged.
+Killing the server flipped the indicator to the red `error` badge and dimmed the table — zero spinners; after a restart the tab returned to `live` and the series continued from the persisted state (`sampleCount` 5, min/max from before the restart preserved).
+The 375px viewport stacked the rows as labeled cards, and SIGINT shut the server down cleanly (exit 130, no exceptions).

docs/plan/README.md

@@ -56,7 +56,7 @@ Server and UI:
 
 - [x] `07-server-mode.md` — `server` subcommand, REST/JSON endpoints, artifact serving
 - [x] `08-web-ui.md` — HTML views with robust live updates
-- [ ] `09-system-metrics.md` — system resource monitoring page
+- [x] `09-system-metrics.md` — system resource monitoring page
 
 Completion: