[flthibaud]

Subscribe checked Exists and then inserted. Two concurrent requests both passed the check and the second hit UNIQUE(user_id, feed_id), which came back as a 500.

• SubscriptionModel.Insert maps that unique violation to data.ErrDuplicateSubscription; Subscribe translates it to ErrAlreadySubscribed (422).
• SubscriptionModel.Exists and its call are removed, and the subscriptionStore interface shrinks.
• New DB test TestSubscriptionInsertDuplicate; the service test now drives the duplicate through Insert.

Tested with gofmt, go vet ./... and go test -race ./... against a migrated postgres:16 (no test skipped).

Closes #23

[flthibaud]

Review: OK to merge.

• Code and tests by Mistral, unchanged.
• The constraint name subscriptions_user_id_feed_id_key is confirmed by the DB test passing on a real schema.
• The orphan-feed cleanup still runs on a failed insert: the deferred DeleteIfOrphan is untouched and keeps its NOT EXISTS guard.
• Not tested: two truly concurrent HTTP requests. The test inserts the duplicate sequentially, which exercises the same constraint.