feat(ai): opt-in structured-system-block caching for one-shot generators (Phase 0.3)

Wraps each static system prompt in a single-block list so Phase 0.1's
AnthropicProvider applies cache_control: ephemeral automatically (policy α,
first block gets marked when no caller-authored cache_control is present).

Call sites:
- ai_tree_generator.scaffold_branches: SCAFFOLD_SYSTEM_PROMPT (~1k tokens)
- ai_tree_generator.generate_branch_detail: BRANCH_DETAIL_SYSTEM_PROMPT
  (~2.5k tokens with few-shot example); retries inside the same function
  re-read the cached block instead of paying full input cost on each attempt
- kb_conversion.convert_document: TROUBLESHOOTING or PROCEDURAL prompt
  (each caches independently by text content)
- ai_fix.generate_fixes: FIX_SYSTEM_PROMPT on first attempt + corrective retry
- script_builder.send_message: SYSTEM_PROMPT_TEMPLATE (per-session language
  substitution — same-language sessions share cache entries)

Each edit includes an inline comment explaining why the block is cacheable
(stable-constant, retry-reuse, per-language variant) so a future dev can
see the intent at the cache_control marker site.

script_builder history caching deliberately deferred — per Phase 0.1
decision (option i), AnthropicProvider does not automatically cache the
message list. If script_builder's growing 20-message history turns out
to be a visible cost driver via the anthropic.cache telemetry, route
that caller through the 0.4 chat wrapper which handles history caching.

No runtime verification from code-server; cache-hit behavior will be
confirmed against the new dev environment when it's up, per the inline
TODO(phase0-verify) in ai_provider.py.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/script_builder_service.py

@@ -220,7 +220,15 @@ async def send_message(
     model = settings.get_model_for_action("script_build")
     provider = get_ai_provider(model=model)
     ai_text, input_tokens, output_tokens = await provider.generate_text(
-        system_prompt=system_prompt,
+        system_prompt=[
+            {"type": "text", "text": system_prompt},
+            # cacheable: SYSTEM_PROMPT_TEMPLATE with a per-session language
+            # substitution. Two sessions on the same language share a cache
+            # entry; different languages cache independently. Conversation
+            # history (ai_messages) is NOT cached at this layer — if that
+            # becomes a cost driver, route script_builder through the chat
+            # wrapper (0.4) which handles history caching.
+        ],
         messages=ai_messages,
         max_tokens=8192,
     )