feat: session file registry with drag-and-drop on day view

Adds a FileDropZone at the bottom of the day view where users can
drop all session files at once. Filenames are regex-matched to
subjects using their animal_id_string (word-boundary aware, so
"2852" won't match "28520"). Only metadata is recorded — files
are never uploaded.

Backend: new session_files table (id, daily_status_id, filename,
file_size BIGINT, file_type, last_modified, notes). Routes:
  POST /api/daily-statuses/:id/files  (batch create)
  GET  /api/daily-statuses/:id/files
  DELETE /api/session-files/:id

Frontend: FileDropZone shows per-subject match groups, unmatched
files, and a warning for subjects with no match. After saving,
registered files are displayed with delete capability.

test-files/: generate_test_files.sh creates 4 dummy files per
subject (2×bin, 2×csv) with correct naming for drag-and-drop
testing. Pre-generated for 2026-04-23 with IDs 2852/3076/3077/3078.

Tests: 19 tests covering drag state, regex matching, save payload,
partial-overlap prevention, multi-subject batching, and delete.

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

test-files/session_files/2026-04-23/2026-04-23_2852_trials.csv

@@ -0,0 +1,21 @@
+trial_num,timestamp,outcome,reaction_time_ms,reward
+1,1.823,Failure,176,0
+2,2.646,Failure,378,0
+3,3.469,Failure,138,1
+4,4.292,Success,150,1
+5,5.115,Success,204,1
+6,5.938,Success,209,0
+7,6.761,Success,369,0
+8,7.584,Failure,84,0
+9,8.407,Failure,403,0
+10,9.230,Success,340,0
+11,10.053,Success,442,1
+12,10.876,Success,355,0
+13,11.699,Success,280,0
+14,12.522,Failure,430,1
+15,13.345,Success,60,1
+16,14.168,Failure,102,0
+17,14.991,Success,213,0
+18,15.814,Success,54,1
+19,16.637,Failure,88,0
+20,17.460,Failure,290,1