FEAT: Video Search In Discovery

discovery/internal/db/migrations/0003_add_video_search.up.sql

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+-- Lexical search over video titles.
+--
+-- The vector is a generated column rather than something the consumer writes:
+-- the catalogue's only writer is SaveVideo, and a title that arrives in an
+-- announcement should be searchable because it is stored, not because a second
+-- statement remembered to index it. STORED because a GIN index needs the value
+-- on disk to index it at all.
+--
+-- 'arabic' is the text search configuration, not 'english' or 'simple': it
+-- stems Arabic (الوثائقي -> وثايق) while leaving Latin-script tokens as written,
+-- which is the right trade for a mixed catalogue. The cost is that English
+-- words are not stemmed, so "documentaries" does not find "documentary".
+--
+-- The configuration is named explicitly rather than left to default_text_search_config,
+-- which is a per-session GUC — an expression that reads it is only STABLE, and
+-- a generated column requires IMMUTABLE. Naming it is also what keeps the
+-- column and every query in step: a query built with a different
+-- configuration would stem its terms differently and silently match nothing.
+ALTER TABLE videos
+    ADD COLUMN search_vector tsvector
+    GENERATED ALWAYS AS (to_tsvector('arabic', title)) STORED;
+
+-- GIN, not GiST: the catalogue is read far more than it is written, and GIN
+-- answers @@ faster at the cost of a slower update — the right way round here.
+CREATE INDEX videos_search_vector_idx ON videos USING GIN (search_vector);