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>

backend/prisma/migrations/20250423000000_add_session_files/migration.sql

@@ -0,0 +1,18 @@
+CREATE TABLE "session_files" (
+    "id"              TEXT         NOT NULL,
+    "daily_status_id" TEXT         NOT NULL,
+    "filename"        TEXT         NOT NULL,
+    "file_size"       BIGINT       NOT NULL,
+    "file_type"       TEXT,
+    "last_modified"   BIGINT,
+    "notes"           TEXT,
+    "created_at"      TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "session_files_pkey" PRIMARY KEY ("id")
+);
+
+ALTER TABLE "session_files"
+    ADD CONSTRAINT "session_files_daily_status_id_fkey"
+    FOREIGN KEY ("daily_status_id")
+    REFERENCES "daily_statuses"("id")
+    ON DELETE CASCADE ON UPDATE CASCADE;