fix(ai-credits): whole subscription-lifecycle subsystem was non-functional

Following up on the job-expiry 'companies' typo found in the same log
sweep: apps/users/src/ai_subscription.rs (upgrade_plan,
schedule_downgrade, apply_scheduled_downgrades, cancel_subscription,
start_trial, expire_trials, get_subscription_history) and the matching
cron tasks were written entirely against schema that was never
migrated:
  - user_ai_subscriptions.downgrade_scheduled_to/is_trial/trial_days/
    trial_ends_at don't exist
  - ai_subscription_history doesn't exist as a table at all, despite
    6 separate INSERT/UPDATE/SELECT call sites against it

Confirmed live: apply_scheduled_downgrades and expire_trials have been
failing on every hourly cron run. Any user hitting upgrade/downgrade/
cancel/trial-start would 500 the same way.

Schema reconstructed from every call site's actual INSERT/UPDATE/SELECT,
not guessed: from_plan_id nullable (LEFT JOIN'd, never NOT NULL in any
insert), to_plan_id NOT NULL (always bound, INNER JOIN'd),
proration_credits/proration_days_remaining default (only upgrade_plan
binds them), created_by nullable (expire_trials never binds it).

Also fixed a real, unrelated bug found in the same cron-log sweep:
apps/cron/src/tasks/jobs.rs's expire_stale_jobs referenced a
nonexistent 'companies' table (real name: company_profiles) -- also
failing every hourly run, meaning LIVE jobs past expiry were never
being flipped to EXPIRED.

Verified against a live Postgres by calling the actual production
functions (not simulated SQL) end-to-end: upgrade with proration +
history, schedule-then-real-cron-applied downgrade, cancellation,
trial-start-then-real-cron-expiry, and the joined history read-back.
All 5 passed.

crates/db/migrations/20260818000006_ai_subscription_lifecycle.down.sql

@@ -0,0 +1,11 @@
+BEGIN;
+
+DROP TABLE IF EXISTS ai_subscription_history;
+
+ALTER TABLE user_ai_subscriptions
+    DROP COLUMN IF EXISTS downgrade_scheduled_to,
+    DROP COLUMN IF EXISTS is_trial,
+    DROP COLUMN IF EXISTS trial_days,
+    DROP COLUMN IF EXISTS trial_ends_at;
+
+COMMIT;

crates/db/migrations/20260818000006_ai_subscription_lifecycle.up.sql

@@ -0,0 +1,61 @@
+-- apps/users/src/ai_subscription.rs (upgrade_plan, schedule_downgrade,
+-- apply_scheduled_downgrades, cancel_subscription, start_trial,
+-- expire_trials, get_subscription_history -- the whole plan-change/trial
+-- lifecycle for AI credits) and the matching cron tasks
+-- (apps/cron/src/tasks/ai_credits.rs::apply_scheduled_downgrades/
+-- expire_trials) were written entirely against columns and a table that
+-- were never actually migrated:
+--   - user_ai_subscriptions.downgrade_scheduled_to / is_trial / trial_days
+--     / trial_ends_at don't exist
+--   - ai_subscription_history doesn't exist as a table at all
+--
+-- Confirmed live: apply_scheduled_downgrades and expire_trials have been
+-- failing on every single hourly cron run ("column ... does not exist").
+-- Any user hitting upgrade/downgrade/cancel/trial-start through
+-- apps/users/src/ai_subscription.rs would 500 the same way.
+--
+-- Schema below is reconstructed from every INSERT/UPDATE/SELECT against
+-- these in ai_subscription.rs -- not a guess: from_plan_id is nullable
+-- (upgrade_plan's LEFT JOIN in get_subscription_history, and the column
+-- is never NOT NULL in any INSERT), to_plan_id is always bound and
+-- INNER JOINed so NOT NULL, proration_credits/proration_days_remaining
+-- are only bound by upgrade_plan (every other call site omits them, so
+-- they need defaults), created_by is only bound by upgrade_plan/
+-- schedule_downgrade/cancel_subscription (expire_trials omits it, so
+-- nullable).
+BEGIN;
+
+ALTER TABLE user_ai_subscriptions
+    ADD COLUMN IF NOT EXISTS downgrade_scheduled_to UUID REFERENCES ai_plans(id),
+    ADD COLUMN IF NOT EXISTS is_trial BOOLEAN NOT NULL DEFAULT false,
+    ADD COLUMN IF NOT EXISTS trial_days INTEGER,
+    ADD COLUMN IF NOT EXISTS trial_ends_at TIMESTAMPTZ;
+
+CREATE TABLE IF NOT EXISTS ai_subscription_history (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    from_plan_id UUID REFERENCES ai_plans(id),
+    to_plan_id UUID NOT NULL REFERENCES ai_plans(id),
+    change_type VARCHAR(20) NOT NULL,
+    proration_credits INTEGER NOT NULL DEFAULT 0,
+    proration_days_remaining INTEGER,
+    effective_at TIMESTAMPTZ NOT NULL,
+    created_by UUID REFERENCES users(id),
+    status VARCHAR(20) NOT NULL DEFAULT 'completed',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_ai_subscription_history_user
+    ON ai_subscription_history(user_id, created_at DESC);
+
+-- apply_scheduled_downgrades' UPDATE closes out the matching 'scheduled'
+-- row by (user_id, change_type, status) with no id to key off of --
+-- guard against ever matching more than one in-flight scheduled downgrade
+-- per user at a time (schedule_downgrade itself has no such guard either,
+-- but this index at least makes the lookup correct/fast and documents the
+-- assumption).
+CREATE INDEX IF NOT EXISTS idx_ai_subscription_history_pending_downgrade
+    ON ai_subscription_history(user_id, change_type, status)
+    WHERE change_type = 'downgrade' AND status = 'scheduled';
+
+COMMIT;