Implement linear task chains and recurring maintenance buckets

Backend, web timeline, and Android widget wiring for the last two
unimplemented items from doot-future-task-scheduling-ideas.

Chains: task_chains table + chain_id/chain_position/chain_unlocked on
native_tasks (migration 026), WIP-limit-1 advancement hooked into
CompleteNativeTask, locked tasks excluded from all date-based queries,
5 new /api/widget/chains* endpoints, an N/M position badge on web and
Android widget rows.

Buckets: maintenance_buckets table + bucket_id/bucket_state/
bucket_last_active_at on native_tasks (migration 027),
staleness-then-priority selection scoring, a new RunBucketCycleCheck
scheduler loop, 5 new endpoints including the distinct Defer action, a
Defer button on web and Android widget rows.

Also corrected stale "not yet approved" status headers on the two
already-shipped specs this work depended on (labels/projects, budgets/
availability) -- their headers were never updated after implementation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

1 files changed, 16 insertions, 0 deletions

diff --git a/migrations/026_task_chains.sql b/migrations/026_task_chains.sql
new file mode 100644
index 0000000..5ed14d7
--- /dev/null
+++ b/migrations/026_task_chains.sql
@@ -0,0 +1,16 @@
+-- Linear task chains: a fixed, ordered sequence of native tasks with a WIP
+-- limit of exactly 1 -- only the task at the current position is ever
+-- unlocked/actionable. Completing it unlocks the next position; completing
+-- the last position marks the chain completed instead.
+CREATE TABLE task_chains (
+    id TEXT PRIMARY KEY,
+    project_id TEXT NOT NULL,
+    status TEXT NOT NULL DEFAULT 'active',
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+ALTER TABLE native_tasks ADD COLUMN chain_id TEXT DEFAULT '';
+ALTER TABLE native_tasks ADD COLUMN chain_position INTEGER DEFAULT 0;
+ALTER TABLE native_tasks ADD COLUMN chain_unlocked BOOLEAN DEFAULT 0;
+
+CREATE INDEX IF NOT EXISTS idx_native_tasks_chain ON native_tasks(chain_id);