Two related search improvements:
1) Your own live-search results now belong to your Mine feed. A new per-user
search_finds table (migration 0030) records each video you surface via your
YouTube search (the route inserts them idempotently). The Mine feed becomes
'your non-hidden subscriptions OR your search finds', and the Source filter
now applies in Mine too: organic = subscriptions, search = your search finds,
all = both (default stays organic, so the main feed is unchanged). The shared
Library keeps using the global via_search flag.
2) Feed search ranks by relevance instead of a whole-phrase substring. A custom
unaccent_simple text-search config + GIN index (migration 0031) back a
YouTube-like fuzzy match: word-order-independent, multi-word AND, prefix on
the word being typed, accent-insensitive. A new 'relevance' sort orders by
ts_rank; the channel name still matches as a substring. The rank is scaled to
an integer so the keyset cursor pages it exactly (a raw float4 breaks paging).
_filtered_query returns the rank expr so only the feed list uses it.
