feat: add the concept of tags

Tags are attached to all magnet links provided by a given source, and
passed to actions. This allows for e.g. better categorization in
Bitmagnet.

migrations/2025-05-04-035709_add_tags/down.sql

@@ -0,0 +1,2 @@
+DROP TABLE magnet_tags;
+DROP TABLE tags;

migrations/2025-05-04-035709_add_tags/up.sql

@@ -0,0 +1,14 @@
+CREATE TABLE tags
+(
+    id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
+    name TEXT UNIQUE NOT NULL
+);
+
+CREATE TABLE magnet_tags
+(
+    magnet_id INTEGER NOT NULL,
+    tag_id INTEGER NOT NULL,
+    FOREIGN KEY (magnet_id) REFERENCES magnets (id),
+    FOREIGN KEY (tag_id) REFERENCES tags (id),
+    PRIMARY KEY (magnet_id, tag_id)
+);