Give tips a date, a scope per child, and a dismissal that sticks

Three faults, all invisible while the tests passed. Advice was built for the
first child only, so a second child got none. The planned days feeding every
count were fetched once per parent and appended to one list, so a couple's
days were counted twice and every intensity and projection derived from them
was wrong. And dismissing a tip forgot the moment the page reloaded.

Tips now carry the date they concern, lifted from figures the rules already
computed, so a view of a month can show only what applies to it. They are
built per child and identified per child, so the same rule firing for two
children is two separate pieces of advice.

A dismissal is stored against a fingerprint of the facts that were dismissed.
When those facts change the advice returns with its new numbers, rather than
one click silencing a warning about a forfeiture that keeps growing.

backend/app/db/connection.py

@@ -173,6 +173,16 @@ CREATE TABLE IF NOT EXISTS day_override (
     UNIQUE(workspace_id, date),
     FOREIGN KEY (workspace_id) REFERENCES workspace(id) ON DELETE CASCADE
 );
+
+CREATE TABLE IF NOT EXISTS tip_dismissal (
+    id TEXT PRIMARY KEY,
+    workspace_id TEXT NOT NULL,
+    tip_id TEXT NOT NULL,
+    data_hash TEXT NOT NULL,
+    dismissed_at TIMESTAMP NOT NULL,
+    UNIQUE(workspace_id, tip_id),
+    FOREIGN KEY (workspace_id) REFERENCES workspace(id) ON DELETE CASCADE
+);
 """