Replace batch DAG system with streaming architecture

- Remove legacy_tasks.py, hybrid_state.py, render.py
- Remove old task modules (analyze, execute, execute_sexp, orchestrate)
- Add streaming interpreter from test repo
- Add sexp_effects with primitives and video effects
- Add streaming Celery task with CID-based asset resolution
- Support both CID and friendly name references for assets
- Add .dockerignore to prevent local clones from conflicting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sexp_effects/effects/blend_multi.sexp

@@ -0,0 +1,58 @@
+;; N-way weighted blend effect
+;; Streaming-compatible: pass inputs as a list of frames
+;; Usage: (blend_multi :inputs [(read a) (read b) (read c)] :weights [0.3 0.4 0.3])
+;;
+;; Parameters:
+;;   inputs      - list of N frames to blend
+;;   weights     - list of N floats, one per input (resolved per-frame)
+;;   mode        - blend mode applied when folding each frame in:
+;;                   "alpha"       — pure weighted average (default)
+;;                   "multiply"    — darken by multiplication
+;;                   "screen"      — lighten (inverse multiply)
+;;                   "overlay"     — contrast-boosting midtone blend
+;;                   "soft-light"  — gentle dodge/burn
+;;                   "hard-light"  — strong dodge/burn
+;;                   "color-dodge" — brightens towards white
+;;                   "color-burn"  — darkens towards black
+;;                   "difference"  — absolute pixel difference
+;;                   "exclusion"   — softer difference
+;;                   "add"         — additive (clamped)
+;;                   "subtract"    — subtractive (clamped)
+;;                   "darken"      — per-pixel minimum
+;;                   "lighten"     — per-pixel maximum
+;;   resize_mode - how to match frame dimensions (fit, crop, stretch)
+;;
+;; Uses a left-fold over inputs[1..N-1].  At each step the running
+;; opacity is:  w[i] / (w[0] + w[1] + ... + w[i])
+;; which produces the correct normalised weighted result.
+
+(require-primitives "image" "blending")
+
+(define-effect blend_multi
+  :params (
+    (inputs :type list :default [])
+    (weights :type list :default [])
+    (mode :type string :default "alpha")
+    (resize_mode :type string :default "fit")
+  )
+  (let [n (len inputs)
+        ;; Target dimensions from first frame
+        target-w (image:width (nth inputs 0))
+        target-h (image:height (nth inputs 0))
+        ;; Fold over indices 1..n-1
+        ;; Accumulator is (list blended-frame running-weight-sum)
+        seed (list (nth inputs 0) (nth weights 0))
+        result (reduce (range 1 n) seed
+                 (lambda (pair i)
+                   (let [acc (nth pair 0)
+                         running (nth pair 1)
+                         w (nth weights i)
+                         new-running (+ running w)
+                         opacity (/ w (max new-running 0.001))
+                         f (image:resize (nth inputs i) target-w target-h "linear")
+                         ;; Apply blend mode then mix with opacity
+                         blended (if (= mode "alpha")
+                                   (blending:blend-images acc f opacity)
+                                   (blending:blend-images acc (blending:blend-mode acc f mode) opacity))]
+                     (list blended new-running))))]
+    (nth result 0)))