feat(llm): adaptive num_ctx tiers + fix KV cache priming num_ctx mismatch

Adds pick_num_ctx() which selects the smallest context window tier
(8192, 16384, 32768) that fits the current messages with 25% headroom,
capped at OLLAMA_NUM_CTX. Threads num_ctx through generation_task.py so
every chat request uses the computed tier rather than a fixed 16384.

Fixes a critical cache miss bug: KV cache priming in app.py and
settings.py was sending requests without num_ctx, so Ollama sized the
cache at its model default (different from the 16384 real requests used),
forcing a full model reload on the first real user message. Both priming
sites now call pick_num_ctx() and pass the matching value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/app.py

@@ -183,16 +183,18 @@ def create_app() -> Quart:
             """Send a minimal chat request to prime Ollama's KV cache with the user's system prompt.
 
             This ensures the next real user message only needs to process its own tokens
-            rather than the full ~5,600-token system prompt, cutting TTFT from ~25s to <1s.
+            rather than the full ~4,650-token system prompt, cutting TTFT from ~25s to <1s.
+            The num_ctx must match what real requests will use so Ollama doesn't reload.
             """
             try:
-                from fabledassistant.services.llm import build_context
+                from fabledassistant.services.llm import build_context, pick_num_ctx
                 messages, _ = await build_context(
                     user_id=user_id,
                     history=[],
                     current_note_id=None,
                     user_message=" ",
                 )
+                num_ctx = pick_num_ctx(messages)
                 async with httpx.AsyncClient(timeout=120.0) as client:
                     await client.post(
                         f"{Config.OLLAMA_URL}/api/chat",
@@ -200,11 +202,11 @@ def create_app() -> Quart:
                             "model": model,
                             "messages": messages,
                             "stream": False,
-                            "options": {"num_predict": 1},
+                            "options": {"num_predict": 1, "num_ctx": num_ctx},
                             "keep_alive": "2h",
                         },
                     )
-                logger.info("Primed KV cache for user %d with model '%s'", user_id, model)
+                logger.info("Primed KV cache for user %d with model '%s' (num_ctx=%d)", user_id, model, num_ctx)
             except Exception:
                 logger.warning("Failed to prime KV cache for user %d", user_id, exc_info=True)