feat(subscriptions): add SaaS subscription tiers, pricing page, and enforced upload quotas

- Add Free / Starter / Professional / Business tiers with lifetime, daily, and monthly
  file limits (app/utils/subscription.py)
- Add subscription_tier column to UserProfile model + migration 014
- Enforce quotas at upload time (HTTP 402 on violation) in /api/ui-upload
- New REST API: GET /api/subscriptions/tiers, /my, /platform (admin)
- New pages: /pricing (marketing, public) and /subscription (per-user status)
- Enhanced dashboard: SaaS stats (files today/month, OCR count, active users)
  in multi-user mode; original single-user layout preserved
- Admin users page: show Plan badge, allow tier editing via dropdown
- Navigation: add Pricing link + subscription icon in user header
- Tests: 23 unit tests for subscription tier logic
- Docs: docs/SubscriptionTiers.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/versions/014_add_subscription_tiers.py

@@ -0,0 +1,30 @@
+"""Add subscription_tier column to user_profiles
+
+Revision ID: 014_add_subscription_tiers
+Revises: 013_add_user_profiles
+Create Date: 2026-03-06
+
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "014_add_subscription_tiers"
+down_revision: Union[str, None] = "013_add_user_profiles"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add subscription_tier column to user_profiles."""
+    op.add_column(
+        "user_profiles",
+        sa.Column("subscription_tier", sa.String(50), nullable=True, server_default="free"),
+    )
+
+
+def downgrade() -> None:
+    """Remove subscription_tier column from user_profiles."""
+    op.drop_column("user_profiles", "subscription_tier")