Feat: Replace UUID image identifiers with 8-character base62 short IDs

Short IDs become the canonical identifier in URLs (/i/:short_id),
MinIO/R2 storage keys, and all API responses. Hash-based deduplication
is preserved. Includes two-phase Alembic migration (003 adds nullable
column, 004 enforces NOT NULL) with a backfill script to copy storage
objects and populate short_id for existing images.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/alembic/versions/004_short_id_not_null.py

@@ -0,0 +1,24 @@
+"""set short_id NOT NULL on images
+
+Revision ID: 004
+Revises: 003
+Create Date: 2026-05-09
+
+IMPORTANT: Run migrate_to_short_ids.py script BEFORE applying this migration.
+This migration will fail if any rows still have short_id IS NULL.
+"""
+
+from alembic import op
+
+revision = "004"
+down_revision = "003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("images", "short_id", nullable=False)
+
+
+def downgrade() -> None:
+    op.alter_column("images", "short_id", nullable=True)