Manage the Slack digest in the admin

New settings area: the webhook lives in the database and is only shown
shortened afterwards, the environment variable stays as a fallback. A switch
decides whether internal entries travel, a button sends the digest right
away, and every run is recorded with period, count, projects, result and
who triggered it.

drizzle/meta/_journal.json

@@ -78,6 +78,13 @@
       "when": 1785579010161,
       "tag": "0010_steady_wither",
       "breakpoints": true
+    },
+    {
+      "idx": 11,
+      "version": "7",
+      "when": 1785751667390,
+      "tag": "0011_concerned_domino",
+      "breakpoints": true
     }
   ]
 }