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_config.py

@@ -1,5 +1,3 @@
-
-
 _BASE_ENV = {
     "DATABASE_URL": "postgresql+asyncpg://u:p@localhost/db",
     "S3_ENDPOINT_URL": "http://localhost:9000",
@@ -26,6 +24,7 @@ def test_settings_load_from_env(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()
@@ -43,6 +42,7 @@ def test_settings_max_upload_bytes_override(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()
@@ -55,6 +55,7 @@ def test_settings_jwt_expiry_override(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()
@@ -67,6 +68,7 @@ def test_api_docs_enabled_default(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()
@@ -79,6 +81,7 @@ def test_api_docs_enabled_false(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()
@@ -91,6 +94,7 @@ def test_api_docs_invalid_value_defaults_to_enabled(monkeypatch):
     import importlib
 
     import app.config as config_module
+
     importlib.reload(config_module)
 
     s = config_module.Settings()