docs(bdd): add AI-routing feature (local-first, BYO fallback)

Gherkin spec for the routing guardrail: local produces the summary by default;
on local failure, fall back only to a user-configured BYO provider; with no BYO,
queue for retry and never send content to a third-party model. Encodes the
local-first/user-owned principle (VISION) as executable behavior.

docs/use-cases/ai_routing.feature

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+Feature: Local-first AI with optional BYO fallback
+  As a user who values keeping my content on my own stack
+  I want summaries produced by the local AI first
+  So that my content only reaches a third-party model when I have explicitly opted in
+
+  Scenario: Local AI produces the summary
+    Given the local AI stack is available
+    When a transcript is summarized
+    Then the local AI produces the summary
+    And the summary records ai_provider "local"
+    And the summary records fallback_used as false
+
+  Scenario: Local AI fails and the user has a BYO provider configured
+    Given the local AI stack fails
+    And I have configured a BYO provider "anthropic"
+    When a transcript is summarized
+    Then Tapir falls back to my "anthropic" provider
+    And the summary records ai_provider "anthropic"
+    And the summary records fallback_used as true
+
+  Scenario: Local AI fails and the user has no BYO provider
+    Given the local AI stack fails
+    And I have not configured any BYO provider
+    When a transcript is summarized
+    Then no summary is produced
+    And the work is queued for retry
+    And my content is not sent to any third-party model
+
+  Scenario: A user without BYO never has content sent externally
+    Given I have not configured any BYO provider
+    When any transcript is summarized
+    Then my content is only ever sent to the local AI stack
+
+  # "Reliably" is operationalized as: Primary returned without error within timeout.
+  # Quality scoring may be added later without changing these scenarios.