-- AppView database schema 0008: notifications + follow-list pagination.
--
-- Why
--
-- Phase 5 shipped the read API (timeline / profile / search / post) but
-- the client had no way to learn that *someone else* interacted with
-- the user: a like, a repost, a follow or a reply never produced a
-- durable record. The Tauri client's tray/notification path (Phase 7)
-- therefore had nothing to poll. This migration adds the table the
-- Jetstream indexer writes into and the read API serves from.
--
-- Table shape
--   id             BIGSERIAL — monotonic tiebreaker for the keyset
--                  cursor. The API's opaque cursor is
--                  `(indexed_at, id)`, mirroring the
--                  `(indexed_at, uri)` pair used by /api/timeline/home,
--                  so the same `routes::cursor` encoder is reused.
--   recipient_did  the user who should SEE the notification (the post
--                  author for like/repost/reply, the followed user for
--                  follow).
--   author_did     the user who CAUSED it (the liker / reposter /
--                  follower / replier).
--   kind           'like' | 'repost' | 'follow' | 'reply'. Enforced by
--                  a CHECK rather than a Postgres ENUM so adding a
--                  variant later is an ALTER ... DROP/ADD CONSTRAINT
--                  instead of a type migration that locks every
--                  dependent object.
--   subject_uri    the post the notification is *about*. NULL for
--                  'follow' (there is no post). For 'like'/'repost'
--                  it's the liked/reposted post (the recipient's own
--                  post); for 'reply' it's the REPLY itself, because
--                  the interesting text to show in the notification
--                  list is what the replier wrote, not what the
--                  recipient already knows they posted.
--   created_at     the interaction's own `createdAt` from the AT record.
--   indexed_at     when WE saw it. This is what the list is ordered by,
--                  for the same reason the timeline orders by
--                  `posts.indexed_at`: a client-supplied `created_at`
--                  can be arbitrarily far in the past or future and
--                  would break keyset pagination.
--   read_at        NULL = unread. Set in bulk by
--                  `POST /api/notifications/seen`.
--
-- Deliberately NO `CHECK (recipient_did <> author_did)`
-- ------------------------------------------------------
-- Self-interactions must not produce notifications, and the indexer
-- enforces that in two places (a pure `should_notify` guard in Rust
-- plus a `WHERE $1 <> $2` in the INSERT ... SELECT). A CHECK would
-- turn a future slip into a constraint violation that aborts the
-- surrounding like/repost transaction — i.e. it would lose the *like*
-- because of a notification bug. Filtering is strictly better than
-- failing here.
--
-- Idempotency
-- -----------
-- `notifications_dedupe_idx` is the unique constraint the indexer's
-- `ON CONFLICT ... DO NOTHING` infers. `subject_uri` is nullable and
-- NULLs never collide in a plain unique index, so the index is on
-- `COALESCE(subject_uri, '')` — that makes the two follow rows
-- (subject_uri IS NULL) for the same (recipient, author) pair collide
-- as intended.
--
-- Consequence worth knowing: unlike-then-relike (or unfollow-then-
-- refollow) does NOT produce a second notification, because the tuple
-- is identical. That is the desired behaviour — it makes notification
-- spam via toggling impossible — but it does mean a notification is
-- "once per (recipient, author, kind, subject)" for all time.

CREATE TABLE notifications (
    id             BIGSERIAL PRIMARY KEY,
    recipient_did  TEXT NOT NULL,
    author_did     TEXT NOT NULL,
    kind           TEXT NOT NULL
                     CHECK (kind IN ('like', 'repost', 'follow', 'reply')),
    subject_uri    TEXT,
    created_at     TIMESTAMPTZ NOT NULL,
    indexed_at     TIMESTAMPTZ NOT NULL DEFAULT now(),
    read_at        TIMESTAMPTZ
);

-- Primary read path: `WHERE recipient_did = $1 ORDER BY indexed_at DESC,
-- id DESC`. The trailing `id DESC` makes the index cover the keyset
-- predicate `(indexed_at, id) < ($2, $3)` end-to-end, so a page fetch
-- never sorts.
CREATE INDEX notifications_recipient_indexed_at_idx
    ON notifications (recipient_did, indexed_at DESC, id DESC);

-- Dedupe / ON CONFLICT target. See the "Idempotency" note above.
CREATE UNIQUE INDEX notifications_dedupe_idx
    ON notifications (recipient_did, author_did, kind, COALESCE(subject_uri, ''));

-- `GET /api/notifications/count` is a hot poll from the client's tray
-- badge, so the unread slice gets its own partial index. It stays tiny
-- because rows leave it as soon as they're marked seen.
CREATE INDEX notifications_unread_idx
    ON notifications (recipient_did)
    WHERE read_at IS NULL;

-- =====================================================
-- follows: pagination indexes for the follower/following lists
-- =====================================================
--
-- `GET /api/followers` and `GET /api/following` page with the same
-- keyset scheme as the timeline: `(indexed_at, <other side's did>)`.
-- The pre-existing indexes cover only the equality half (the PK covers
-- `follower_did`, `follows_subject_idx` covers `subject_did`), which
-- leaves Postgres sorting the whole follower set on every page. These
-- two make both directions index-ordered.
CREATE INDEX IF NOT EXISTS follows_subject_indexed_at_idx
    ON follows (subject_did, indexed_at DESC, follower_did DESC);

CREATE INDEX IF NOT EXISTS follows_follower_indexed_at_idx
    ON follows (follower_did, indexed_at DESC, subject_did DESC);