Reorganize into src/ with script/assets/templates, illustration/ at root

New layout:
  src/script/app/    Python package (was app/)
  src/assets/img/    SVGs (was static/*.svg)
  src/assets/css/    stylesheet (was static/style.css)
  src/templates/     HTML templates (was templates/)
  illustration/      README screenshot (was screenshots/), kept at
                      the repo root since it's a doc asset, not
                      something the app serves
  main.py, jobs/, VERSION, docker/  unchanged locations

main.py stays at the repo root as the entry point (matches how
Docker and local runs already invoke it) and prepends src/script to
sys.path before importing app, rather than moving into src/script/
itself and needing every entry point updated to match.

config.py's path constants are recomputed for app/config.py's new
depth (src/script/app/), with SRC_DIR and REPO_ROOT resolved
explicitly rather than a single ambiguous BASE_DIR, since "templates
and assets live under src/" and "jobs and VERSION live at the repo
root" are no longer the same directory. Flask's static_folder now
points at src/assets, which serves img/ and css/ under it at
whatever URL prefix Flask derives from the folder name (/assets, not
/static); templates already reference assets via the "static"
endpoint name through url_for, so this needed the url_for() calls'
filename argument updated with the img/ or css/ prefix, not a
hardcoded URL change.

Docker copies src/ into the image preserving this same internal
structure, so the path arithmetic in config.py resolves identically
in both a local checkout and the container, no --chdir or other
container-specific path handling needed.

Verified end to end after the move: local Flask dev server, a full
Docker rebuild, and a real HTTP conversion request through the
running container (TIFF HDR pipeline, numpy/tifffile imports
included) all succeed.

main.py

@@ -8,18 +8,21 @@ LICENSE). Gives you a browser form instead of a terminal, and
 surfaces the assembly report (including the "GainMapMin is negative"
 warning) on a results page.
 
-This file is just the entry point — see app/ for the app itself:
-  app/config.py       constants
-  app/jobs.py         in-memory job registry + heartbeat/reaper
-  app/validation.py   upload + Instagram resolution checks
-  app/assembler.py    gain-map assembly pipeline
-  app/routes.py       Flask routes
+This file is just the entry point — see src/script/app/ for the app itself:
+  src/script/app/config.py       constants
+  src/script/app/jobs.py         in-memory job registry + heartbeat/reaper
+  src/script/app/validation.py   upload + Instagram resolution checks
+  src/script/app/assembler.py    gain-map assembly pipeline
+  src/script/app/routes.py       Flask routes
 
 Run directly:      python3 main.py
 Run in Docker:      see Dockerfile (gunicorn main:app)
 """
 import os
 import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent / "src" / "script"))
 
 from app import create_app
 from app.assembler import check_dependencies