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/tests/unit/test_hashing.py

@@ -1,6 +1,6 @@
 import hashlib
 
-from app.utils import compute_sha256
+from app.utils import compute_sha256, generate_short_id
 
 
 def test_sha256_known_bytes():
@@ -19,3 +19,24 @@ def test_sha256_returns_64_char_hex():
     result = compute_sha256(b"test data")
     assert len(result) == 64
     assert all(c in "0123456789abcdef" for c in result)
+
+
+def test_generate_short_id_length():
+    assert len(generate_short_id()) == 8
+
+
+def test_generate_short_id_charset():
+    result = generate_short_id()
+    assert all(
+        c in "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" for c in result
+    )
+
+
+def test_generate_short_id_randomness():
+    assert generate_short_id() != generate_short_id()
+
+
+def test_generate_short_id_importable():
+    from app.utils import generate_short_id as fn
+
+    assert callable(fn)