feat: add tag key prefix queries for namespace-scoped discovery

[deepjoy]

Summary

• Add four new query methods (tag_keys_by_prefix, tasks_by_tag_key_prefix, count_by_tag_key_prefix, cancel_by_tag_key_prefix) across TaskStore, Scheduler, ModuleHandle, and DomainHandle for discovering and operating on tasks by tag key prefix (e.g. "billing.")
• Add escape_like_prefix() helper that escapes SQL LIKE wildcards (%, _, \) in user-supplied input before appending the trailing %, ensuring only true prefix matching
• Add domain-scoped _with_prefix store variants that additionally filter by task_type LIKE 'domain%', consistent with the existing tag query pattern
• Document the new APIs in crate-level docs, docs/query-apis.md, and docs/multi-module-apps.md

Motivation

When multiple independent libraries share a single scheduler, each naturally namespaces its tags with a prefix (billing.customer_id, media.pipeline, etc.). The existing exact-match tag API cannot answer namespace-scoped questions like "how many billing tasks are active?" without knowing every possible billing.* key upfront. The new prefix queries fill this gap.

Closes #41

[github-actions]

Benchmark Comparison

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group                                       current
-----                                       -------
backoff_delay/constant                      1.00     44.0±2.13ns        ? ?/sec
backoff_delay/exponential                   1.00    190.6±0.50ns        ? ?/sec
backoff_delay/exponential_jitter            1.00    407.7±3.21ns        ? ?/sec
backoff_delay/linear                        1.00     76.3±0.22ns        ? ?/sec
batch_submit_1000                           1.00     33.5±2.62ms        ? ?/sec
byte_progress/byte_reporting_500            1.00    242.9±4.50ms        ? ?/sec
byte_progress/noop_500                      1.00    242.8±4.49ms        ? ?/sec
byte_progress_snapshot_100_tasks            1.00     80.3±3.35ms        ? ?/sec
concurrency_scaling/1                       1.00    376.7±5.83ms        ? ?/sec
concurrency_scaling/2                       1.00    312.7±5.54ms        ? ?/sec
concurrency_scaling/4                       1.00    269.4±5.19ms        ? ?/sec
concurrency_scaling/8                       1.00    240.6±4.14ms        ? ?/sec
count_by_tags/100                           1.00    127.8±2.76µs        ? ?/sec
count_by_tags/1000                          1.00    218.7±8.37µs        ? ?/sec
count_by_tags/5000                          1.00   625.1±16.02µs        ? ?/sec
dep_chain_dispatch/10                       1.00     11.1±0.12ms        ? ?/sec
dep_chain_dispatch/25                       1.00     26.6±0.48ms        ? ?/sec
dep_chain_dispatch/50                       1.00     52.9±0.98ms        ? ?/sec
dep_chain_submit/10                         1.00      3.3±0.15ms        ? ?/sec
dep_chain_submit/200                        1.00     76.8±3.70ms        ? ?/sec
dep_chain_submit/50                         1.00     17.4±0.98ms        ? ?/sec
dep_fan_in_dispatch/10                      1.00      7.5±1.12ms        ? ?/sec
dep_fan_in_dispatch/100                     1.00     52.5±0.80ms        ? ?/sec
dep_fan_in_dispatch/50                      1.00     27.3±0.64ms        ? ?/sec
dispatch_and_complete_1000                  1.00    485.4±8.33ms        ? ?/sec
dispatch_group_scaling/1                    1.00    409.2±5.12ms        ? ?/sec
dispatch_group_scaling/10                   1.00    408.5±5.42ms        ? ?/sec
dispatch_group_scaling/100                  1.00    410.0±7.45ms        ? ?/sec
dispatch_group_scaling/50                   1.00    408.8±6.00ms        ? ?/sec
dispatch_no_groups_500                      1.00    242.0±4.93ms        ? ?/sec
dispatch_one_group_500                      1.00    404.8±6.44ms        ? ?/sec
dispatch_permanent_failure_500              1.00    360.0±7.48ms        ? ?/sec
history_by_type/100                         1.00   935.2±26.55µs        ? ?/sec
history_by_type/1000                        1.00   948.6±29.18µs        ? ?/sec
history_by_type/5000                        1.00   943.1±19.73µs        ? ?/sec
history_query/100                           1.00   545.7±14.49µs        ? ?/sec
history_query/1000                          1.00    549.2±6.06µs        ? ?/sec
history_query/5000                          1.00    561.9±7.68µs        ? ?/sec
history_stats/100                           1.00    145.7±1.13µs        ? ?/sec
history_stats/1000                          1.00    357.6±5.48µs        ? ?/sec
history_stats/5000                          1.00   1284.4±4.85µs        ? ?/sec
mixed_priority_dispatch_500                 1.00    269.3±4.93ms        ? ?/sec
peek_next/100                               1.00    118.4±2.10µs        ? ?/sec
peek_next/1000                              1.00    118.8±2.68µs        ? ?/sec
peek_next/5000                              1.00    119.1±5.87µs        ? ?/sec
query_by_tags/100                           1.00  1288.1±175.75µs        ? ?/sec
query_by_tags/1000                          1.00     10.8±1.57ms        ? ?/sec
query_by_tags/5000                          1.00     57.3±8.03ms        ? ?/sec
retryable_dead_letter/constant              1.00    146.3±3.30ms        ? ?/sec
retryable_dead_letter/exponential           1.00    146.2±4.59ms        ? ?/sec
retryable_dead_letter/exponential_jitter    1.00    146.2±1.94ms        ? ?/sec
retryable_dead_letter/linear                1.00    147.8±4.69ms        ? ?/sec
submit_1000_tasks                           1.00    180.5±5.69ms        ? ?/sec
submit_dedup_hit_1000                       1.00   240.5±10.28ms        ? ?/sec
submit_with_tags/0                          1.00     89.8±2.83ms        ? ?/sec
submit_with_tags/10                         1.00   240.4±10.95ms        ? ?/sec
submit_with_tags/20                         1.00   389.6±16.14ms        ? ?/sec
submit_with_tags/5                          1.00    165.6±6.50ms        ? ?/sec
tag_values/100                              1.00    133.6±3.56µs        ? ?/sec
tag_values/1000                             1.00    193.8±3.61µs        ? ?/sec
tag_values/5000                             1.00    458.2±4.52µs        ? ?/sec