feat: profile bio

migrations/0047_add_users_bio.down.sql

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+ALTER TABLE users
+    DROP COLUMN IF EXISTS bio;

migrations/0047_add_users_bio.up.sql

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+-- Profile bio — a short self-introduction shown on the profile header.
+--
+-- Plain text (never HTML): `@handle` mentions are stored verbatim and linkified
+-- at render time by the frontend, so nothing user-supplied is ever trusted as
+-- markup. NULL means "no bio set", which is distinct from an empty string —
+-- the update path clears to NULL rather than storing ''.
+--
+-- The 200-character cap is enforced in the service layer (by rune count, so a
+-- devotee writing in Devanāgarī gets the same 200 characters a Latin-script
+-- writer does). No CHECK constraint here: the column stays permissive so a
+-- future cap change doesn't need a migration on a large table.
+ALTER TABLE users
+    ADD COLUMN IF NOT EXISTS bio TEXT;