fix: model-loaded guards on transcribe_pcm, power assertion guards on tasks LLM commands

transcribe_pcm and transcribe_pcm_parakeet did not check that their
respective engines were loaded before clone+spawn_blocking. transcribe_file
already calls ensure_model_loaded; these now mirror that posture with a
friendly error when the engine is unloaded, matching what
extract_content_tags_cmd does.

decompose_and_store and extract_tasks_from_transcript_cmd ran multi-second
LLM inference inside spawn_blocking without the PowerAssertion guard that
cleanup_transcript_text_cmd and extract_content_tags_cmd already use. Both
now begin a guard so the macOS App-Nap inhibitor (and the planned
Linux/Windows equivalents per KI-02, KI-03) can pin the process for the
duration of the inference.

HANDOVER.md gets a status note clarifying it captures Phase 9 state. The
schema head referenced (v14) was the head at session time; current head
is v15 (`idx_transcripts_profile_created` composite index) per the
architecture map.

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

HANDOVER.md

@@ -7,6 +7,8 @@ description: Session handover — 2026/04/24-25 Phase 9 polish debt mostly shipp
 
 # Magnotia Handover — 2026/04/25
 
+> **Note:** Session-specific handover. Migration v14 (`transcripts.llm_tags`) was the head **at the time of this session**. Subsequent work has advanced the schema; current head is v15 (`idx_transcripts_profile_created`, composite index). For the current codebase shape, see [`docs/architecture-map/`](docs/architecture-map/) and [`KNOWN-ISSUES.md`](KNOWN-ISSUES.md).
+
 Phase 9 session. Spec + plan written from scratch and committed; plan corrections layered in after critical review against the actual codebase (Codex was unreachable for cross-model review, three retries failed at the ChatGPT-account-entitlement layer). Sub-phases 9a + 9b + sparkline polish landed end to end. Sub-phase 9c reduced to the Phase 8 carryover bug fix; sub-phase 9d's walkthrough sweeps deferred to Phase 10a QC.
 
 ## Rebrand note