feat(stats): split repos public/private + 7-day daily commit series

Collector now reports repos_public/repos_private (repos kept as the sum)
and a commits_daily[] series of 7 {date,commits} buckets (UTC days,
zero-filled), bucketed in Go from raw created_unix to avoid MySQL
session-timezone ambiguity. commits_7d is now the sum of those buckets
so the card total matches the sparkline.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -12,8 +12,33 @@ A tiny Go service that periodically snapshots Gitea instance metrics from a
 metrics card. No HTTP endpoint is exposed to browsers; a local-only
 `/healthz` is provided for ops.
 
-Metrics: repositories, claw agents (non-admin users), commits in the last
-7 days, pull requests, merged pull requests.
+Metrics: repositories (split public/private), claw agents (non-admin
+users), commits in the last 7 days (total plus a per-day series over the
+last 7 UTC calendar days), pull requests, merged pull requests.
+
+### Output schema (`stats.json`)
+
+```json
+{
+  "repos": 42,
+  "repos_public": 30,
+  "repos_private": 12,
+  "agents": 7,
+  "commits_7d": 91,
+  "commits_daily": [
+    {"date": "2026-05-22", "commits": 10},
+    {"date": "2026-05-23", "commits": 14}
+    // … 7 entries total, oldest → newest (UTC days)
+  ],
+  "prs": 18,
+  "merged": 12,
+  "generated_at": "2026-05-28T00:00:00Z",
+  "interval_hours": 12
+}
+```
+
+`commits_7d` equals the sum of `commits_daily` (7 calendar days, UTC), so
+the card total always matches the sparkline.
 
 ### Configuration (all via environment variables — no secrets in the image)
 
@@ -31,7 +56,7 @@ refresh it keeps the previous output (stale-while-error).
 
 ```sh
 # from repo root
-IMAGE=git.hangman-lab.top/hzhang/gitea-stats TAG=0.1.0 ./publish.sh
+IMAGE=git.hangman-lab.top/hzhang/gitea-stats TAG=0.2.0 ./publish.sh
 ```
 
 ### Run
@@ -42,7 +67,7 @@ docker run -d --name git-kc-stats \
   -e STATS_DB_DSN='gitea_ro:***@tcp(mysql:3306)/giteadb?timeout=5s&readTimeout=10s&loc=UTC' \
   -e STATS_INTERVAL=12h \
   -v /var/lib/gitea/custom/public/assets:/out \
-  git.hangman-lab.top/hzhang/gitea-stats:0.1.0
+  git.hangman-lab.top/hzhang/gitea-stats:0.2.0
 ```
 
 > The read-only MySQL user (`gitea_ro`, `SELECT` only on `giteadb`) and its