fix(whisper): avoid repetition loops on long audio, add startup warmup

[eksrha]

Closes #17

What

• Temperature: without an explicit temperature form field the server now passes faster-whisper's fallback schedule (0.0, 0.2 ... 1.0, default compression-ratio/log-prob/no-speech thresholds) instead of the single float 0.0. An explicit value (including 0) is used as given.
• condition_on_previous_text is off by default (whisper.conditionOnPreviousText / WHISPER_CONDITION_ON_PREVIOUS_TEXT).
• Audio longer than WHISPER_BATCH_THRESHOLD_S (default 35 s, chart whisper.batchThresholdSeconds) goes through BatchedInferencePipeline (WHISPER_BATCH_SIZE, default 8, whisper.batchSize). Audio is decoded once, the duration comes from the decoded samples. Shorter audio uses the regular transcribe().
• Warmup: after loading the model a 2 s synthetic clip runs through VAD and decoder (WHISPER_WARMUP, default true); /health returns 503 until model and warmup are done. A failed warmup is only logged.
• whisper_request log gains path (standard|batched), batch_size, temperature_mode (fallback|fixed), condition_on_previous_text. Prompt/hotwords logic and response formats are unchanged.

Notes

• The batched pipeline in faster-whisper 1.2.1 always needs VAD and uses only the first temperature; this is documented in the README.
• Clients that always send temperature=0 get no fallback on the standard path (explicit value wins).

Measured locally (large-v3-turbo, int8, 4 threads, beam 1, VAD, FLEURS de clips)

• Three 55-88 s clips: no repetition loop (88 s clip: 132 words out for 132 reference words, previously 256 vs 120), WER about 3 % on long clips instead of 42 %.
• 88 s clip in 23.5 s, 56 s clip in about 16 s (previously 27 s); short clips unchanged (about 8 s for 8.7 s of audio).
• First request after start: 8.4 s without warmup, 7.7 s with it. Warmup itself takes about 8 s at startup.

Tests

pytest (stubbed model): path selection, threshold, temperature fallback vs explicit, condition flag, log fields, response formats on both paths, health/warmup lifecycle.