feat(oidc): support of +alias email (clean it up to reconciliate)

migrations/20261011000000_users_normalized_email_index.sql

@@ -0,0 +1,44 @@
+-- Identity-lookup email column + b-tree index. Powers
+-- `list_users_by_normalized_email`, the auto-link ambiguity detector
+-- added alongside the OIDC-linking work
+-- (see docs/plan/oidc-account-linking.md § Auto-link).
+--
+-- Normalization matches `common::text::normalize_email_for_link`:
+-- lowercase + strip `+alias` sub-addressing from the local part.
+-- Unconditional storage: every row carries the normalized form, even
+-- when it equals `email`. The alternative (only populate when the
+-- normalized form differs, then `WHERE email = $1 OR
+-- identity_lookup_email = $1`) saves a few bytes per row but forces
+-- a two-branch lookup on every call. Storage cost is minimal
+-- (~30 bytes/user); simplicity of the always-populated lookup wins.
+--
+-- Storing the normalized form as its own column (rather than doing
+-- the computation in the WHERE clause) buys three things:
+--   1. Plain-equality SQL — the lookup is a one-liner, not a
+--      SPLIT_PART/LOWER expression tree that's easy to mis-copy.
+--   2. Debuggable — operators can `SELECT username, email,
+--      identity_lookup_email FROM auth.users` and immediately see
+--      why two users collide under normalization.
+--   3. Automatic — GENERATED ALWAYS AS ... STORED means PostgreSQL
+--      itself keeps the column in sync on every INSERT/UPDATE of
+--      `email`. No trigger, no application code, no drift risk.
+--
+-- Table rewrite cost: ALTER TABLE ADD COLUMN with a GENERATED
+-- expression forces a full-table rewrite (each row needs the
+-- computed value stored). Brief, exclusive lock. Fine at OxiCloud's
+-- expected sizes (self-hosted, hundreds to tens of thousands of
+-- users); would need a batched backfill on a million-row deployment.
+ALTER TABLE auth.users
+ADD COLUMN identity_lookup_email TEXT
+GENERATED ALWAYS AS (
+    LOWER(
+        SPLIT_PART(SPLIT_PART(email, '@', 1), '+', 1)
+        || '@'
+        || SPLIT_PART(email, '@', 2)
+    )
+) STORED;
+
+-- Regular b-tree index on the stored column. O(log n) probes for the
+-- auto-link ambiguity check on the OIDC callback path.
+CREATE INDEX idx_users_identity_lookup_email
+    ON auth.users(identity_lookup_email);