feat(core): first-class tasks + task_events entity and relay API

[mfethe1]

Summary

Adds the first two phases of Buzz's community-scoped task system: the durable task/task-event backend and mobile quick actions for creating tasks and summarizing threads into tasks.

Phase 1 — task entity and relay API

• Added additive migration 0032_task_system.sql and schema/schema.sql parity for tasks and task_events.
• Added tenant-scoped task status, priority, parent, assignee, source, and completion invariants.
• Added the task lifecycle event log with a single-summary constraint.
• Added authenticated task create/list/get/update and task-event endpoints with host-derived community scoping and channel access filtering.
• Restricted event writes to the supported lifecycle actions; transition events are derived by the API rather than accepted as forged input.

Phase 2 — mobile quick actions

• Added a + Task composer action that opens the New task sheet with channel/thread source context.
• Added mobile task models/API and the New task sheet, including scope selection, optional assignee mention, due-date presets, priority, validation, and relay error handling.
• Added a thread-header Summarize thread action.
• Added a deterministic extractive Markdown digest that excludes system/blank rows, preserves thread order, highlights decisions and open questions, and extracts links.
• Added copy-to-clipboard and save-to-task flows for a new or existing task; summaries persist through the supported summary_persisted task event.

Verification

All commands were run from this worktree with the repository Hermit toolchain.

• dart format --output=none --set-exit-if-changed . — exit 0; 442 files checked, 0 changed.
• flutter analyze --no-fatal-infos — exit 0; no issues found.
• flutter test — exit 0; all 1,638 tests passed.
• just fmt-check — exit 0.
• just clippy — exit 0; workspace/all-targets clippy passed. Local Linux OpenSSL was supplied through PKG_CONFIG_PATH because the environment did not expose openssl.pc by default.
• just test-unit — exit 0; unit-test recipe passed.
• Focused Phase 2 tests — exit 0; 83 tests passed.
• DCO Check — pass.

Scope note

The mobile tests use mocked HTTP clients for the task flows. A live relay/HTTP integration run was not performed in this verification pass.

[mfethe1]

Rebased onto main (040b203f7) and pushed as a2bd15f8e. This PR was mergeable=false / dirty; it is now mergeable=true. Two things needed resolving, and the second was not visible to git.

1. App bar conflict in mobile/lib/features/channels/thread_detail_page.dart

main rewrote this FrostedAppBar — IosGlassNavigationButton leading, iconColor, and a padded title carrying ValueKey('thread-app-bar-title'). This branch had only made the title Text const, which upstream's rewrite supersedes, so upstream's version is kept whole and the const-title line is dropped.

This branch's actual contribution to that widget — the actions: [_SummarizeThreadButton(...)] block and the thread_summary imports — merged cleanly outside the conflict region and is unchanged. Verified post-merge that both the glass back button and _SummarizeThreadButton are present.

2. A duplicate migration version that merged cleanly

This is the part worth flagging. main landed 0032_channel_roster_snapshot_fence.sql (#6251) while this branch carried 0032_task_system.sql. Different filenames, so git merged them without a conflict and produced two migration version 32s. git merge-tree reported zero conflicts against a tree that was semantically broken.

The symptom was a test on this branch silently asserting against the wrong migration: assert_eq!(migrations[31].version, 32) had begun reading upstream's roster fence rather than the task system, and failed on sql.contains("CREATE TABLE tasks").

Fixed by renumbering ours, since upstream owns 0032:

• migrations/0032_task_system.sql → migrations/0033_task_system.sql — renamed, not rewritten. No migration SQL changed, so brownfield checksums are untouched and the additive-migration invariant holds.
• task_system_tables_are_additive_tenant_scoped_and_fenced now reads migrations[32] / version 33.
• embedded_migrator_contains_consolidated_initial_schema now expects 33 migrations. git blame confirms that line is this branch's own (40f39053de bumped it 31→32); upstream's addition makes 33 correct. On-disk count verified: 33 here, 32 on main.
• Renumbered the stale 0032 comments in crates/buzz-db/src/task.rs and schema/schema.sql.

No assertion was weakened to make anything pass. Every line changed was introduced by this branch.

Verification

suite	result
cargo test -p buzz-db --lib	109 passed, 0 failed
cargo test -p buzz-core --lib	269 passed, 0 failed
cargo test -p buzz-relay --lib tasks	8 passed, 0 failed
cargo test -p buzz-relay --lib workflow_sink	17 passed, 0 failed
duplicate migration versions	none

cargo test -p buzz-relay --lib (full) is 906 passed / 8 failed. Those 8 are not from this branch — I ran the same suite in a worktree on untouched main (040b203f7) and diffed the failure name sets:

ours:     906 passed, 8 failed
baseline: 898 passed, 9 failed   (main, untouched)
ours minus baseline:  empty      (zero regressions)

All 8 (api::media x6, api::admin x2) fail identically on main and reproduce under --test-threads=1, so they are pre-existing and environmental rather than order-dependent. The +8 pass delta is exactly this branch's new task tests.

Baseline additionally fails telemetry::tests::trace_context_lookup_does_not_enable_callsites. That one is genuinely order-dependent global tracing state — on this branch it failed 1 of 3 identical full-suite runs. I'm noting it as flaky rather than claiming this branch fixed it.

Not verified locally

dart/flutter are not installed on this machine, so the Dart edit above is not locally compiled or analyzed. It is structurally sound and marker-free, but "it builds" is not something I have evidence for — mobile CI is the first real check. Flagging that explicitly rather than implying a green mobile gate.

Since this touches migrations/ and schema/schema.sql, I'm treating it as review-required regardless of gate colour and am not self-merging.