Add last_detected_at and last_matched_at tracking

Track when cookies are last detected (last_detected_at on cookies)
and when patterns last matched a detected cookie
(last_matched_at on cookie_patterns). The cookie timestamp is
refreshed on every detection report; the pattern timestamp is
computed as MAX(last_detected_at) during pattern analysis.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/migrations/20260504T114755Z.sql

@@ -0,0 +1,26 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE cookies ADD COLUMN last_detected_at TIMESTAMPTZ NOT NULL DEFAULT now();
+UPDATE cookies SET last_detected_at = created_at;
+ALTER TABLE cookies ALTER COLUMN last_detected_at DROP DEFAULT;
+
+ALTER TABLE cookie_patterns ADD COLUMN last_matched_at TIMESTAMPTZ;
+UPDATE cookie_patterns SET last_matched_at = sub.max_detected
+FROM (
+    SELECT cookie_pattern_id, MAX(last_detected_at) AS max_detected
+    FROM cookies
+    GROUP BY cookie_pattern_id
+) sub
+WHERE cookie_patterns.id = sub.cookie_pattern_id;