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.

README.md

@@ -7,14 +7,14 @@
 A small web front end for assembling Instagram-compatible HDR JPEGs (gain
 map) from an SDR export and an HDR export out of Lightroom or Camera Raw.
 
-The gain-map assembly logic ([app/assembler.py](app/assembler.py)) is
+The gain-map assembly logic ([src/script/app/assembler.py](src/script/app/assembler.py)) is
 adapted from [kostis-kounadis/instagram-hdr-assembler](https://github.com/kostis-kounadis/instagram-hdr-assembler),
 itself based on the reverse-engineering work of
 [karachungen/instagram-hdr-converter](https://github.com/karachungen/instagram-hdr-converter).
 Original MIT license kept in [LICENSE](LICENSE). 
 
 
-![Instagram HDR Assembler screenshot](screenshots/insta-hdr-1.png)
+![Instagram HDR Assembler screenshot](illustration/insta-hdr-1.png)
 
 ## How it works